After over 3years of beating my head against the wall trying to get this basic WordPress site to do what I want it to, I’ve bit the bullet and moved to my own address. If I wasn’t so cheap, I’d probably set up some DNS to auto mask the address but all my funds are trying to keep my proprietary software in check.

Good news is that I have moved to HTTP://DRAGONS8MYCAT.COM which is slightly more shiny but just as useful. I look forward to seeing you there soon!

Roger Tomlinson


The #GISTribe Challenge


So, after a lot of talk with @WildlifeGISgirl, @UUDreams, @Brian_Bancroft & the plethora of super geoninja on the #GISTribe forum (is it a forum, a meet, a symposium or even a tweeting?)…..I think I have the happenings of an outline for a challenge but feel free to comment and advise on how this can be made better. Here is my brain dump on how I think this could work;

The rules of running the challenge:

1. The challenge is monthly

2. The challenge is hosted by someone different

3. The challenge must be able to be done cross platform so that anyone (and I include my 7yr old daughter in this) can do it

4. The subject matter should be something that betters or helps the community (GIS or humanity)

5. All data/resources should be open source and in the public domain for ease of use (and legal!)

Does this sound fair? I am guessing that if we stick to these basic rules that we can add/remove more later if the game expands…One thing that I, personally, am keen on is introducing people to new data and new ways of working with it…so introducing people to the great open data from across the globe to solve interesting problems.

So here it is:

I was originally going to do a challenge based on cumulative impacts of offshore infrastructure and shipping within the United States and how it would affect other countries, though I thought that it would be a little mean for a first challenge. Luckily or rather I SHOULD say unfortunately, something else has dominated the news, Ebola….so this will be the challenge for this month.

The Brief:

Ebola is currently the word on everyones lips, in September, the World Health Organisation released information that the first human to human transmission of the disease had occured outside of Africa (Article here).

Objective: Using the resources provided, create a map which can be used to inform the public of the Ebola situation

The product can be of your choosing, you could provide a historic map of Ebola cases, you could provide a map of the increase in cases, you could even create a timeline map or a 3D flythrough of Africa with people waving and saying hello (as long as it relates to Ebola)….or, of course, you could produce something of your own which would help disseminate information about Ebola (using geo as a base)

End Date

All entries should be submitted on the #GISTribe site (or for PDF & other formats to by SUNDAY 9th NOVEMBER 2014.

This will give the judges time to review all entries prior to the result on #GISTribe the following Wednesday during the weekly hangout/meet/tweeting etc,,,

Data Sources:

Of course, to do this challenge you will require the necessary Ebola data, this can be found below;

Historical Ebola information: Data Here

Situation Reports on Ebola: Data Here

Here are the world administritive boundaries and other useful vectors for this via Natural Earth: Data Here

Anything Else?

Hopefully, that should be straight forward and easy to understand some simple rules can be found here, but if you have any input or questions, please feel free to contact me on Twitter: @Dragons8mycat


Nick D

Can I use that geospatial data or map?

Originally posted on 13th August 2o14:

It’s no big secret, anyone can make maps, ESRI even had some children showcasing their abilities at the user conference this year, So why employ a GIS specialist? Well, apart from getting the cartography right, the specialist can advise on the legalities of map & data production.

With data becoming ever more available and online maps being the norm, there is now more copyright infringement than ever before. In some workplaces it is common to see Google Earth imagery & Streetview imagery put into reports as screen grabbed images….you do know that this is a breach of use?

With everything being so accessible nowadays it is easy to see something relevant and screen grab it, then make it available to others, but have you ever stopped to think about whether you can?

It doesn’t seem like a large number but 13% of all Government Google Removal Requests have come from court orders for copyright offences (figures here) & there have been over 27,662,690 URL removal requests to Google due to copyright (figures here). Okay, so a lot of the URL removals are music related, music piracy is a huge problem. But this highlights the breadth and size of the issue at hand and the rate at which companies are being reported and fined.

Law is a complicated beast, so to help simplify things I put together a flowchart to help end users come to the right decision.

©Nicholas Duggan 2014

Flowchart of the questions you should ask before using data or maps

Although it does not cover the plethora of data or map licence types, this chart provides an easy reference as to whether you may or may not use the material you intend to use. Of course this may vary from country to country and on a case by case basis, also this does not serve as a legal document, legal advice should be obtained in case of dispute.




The thing that is damaging the GIS industry the most…..

A while back (names will be changed to protect the not so innocent) I was at a conference and bumped into an ESRI employee & introduced myself as Dragons8mycat, they responded by saying:

“You’re that guy that hates ESRI aren’t you?”

This got my heckles up and is what is fundamentally wrong with the GIS industry at the moment, it’s like a war, you are either in one camp or another and this should NEVER be the case, to get good at something you don’t read just one source, you don’t stick with one TV channel do you?…..

No, to learn and perform the best we can in life, we use MANY different sources. So why am I ridiculed for not liking parts or the services of software? Let’s be hones here, every single GIS on the market has its issues; ArcGIS crashes & is very pricey, QGIS (although 150% better than it was 3yrs ago) has a complicated GUI and lacks some basic features, CADCorp lacks proper 3D, MapInfo has transformation issues…….let’s not get me started on Tatuk.

At the same time there is a lot to love about every single one of those software and to get the job done I would have every single one of them on my machine; ArcGIS can do ANY geoprocess I need to do, QGIS is fast, reliable and integrates well with web development, CADCorp is fast, easy to use & has amazing support, MapInfo….is MapInfo…..but you can see where I am coming from.

Instead of fighting over which is best, segregating those who dare complain & creating systems with amazing features that turn back time or generate maps by retina recognition, the creators should look at what people are saying and fix what people are complaining about.

Excuse me for saying this but it is an industry in-joke that ArcGIS crashes constantly, since ArcMap 8.x there have been calls for an autosave feature, so why, at the pinnacle of GIS technology is there still online talk of constant crashes?……

Before you ask, I am impartial, I love all the GIS out there at present, each one of them has something great to shout about, if someone asked me what they should have in their office, at present I would be hard pushed to choose one, it would be a business case decision…..

This is me begging those developers and managers of the major GIS companies out there to stop bickering and take a look at yourselves, take the knocks on the chin and listen to what the public are saying (and not the bias devs around you).

I love you guys, all of you, but you’re giving me a headache…if you carry on like this someone is going to build a system that ticks all the boxes and I need you in my life too much to let that happen.



GIS Tips: GIS Tools of the trade

Still relevant…..

The Spatial Blog

Today I got asked by someone starting out in a new GIS job at a Local Council, what software they needed, to which, my answer is always “It depends on what you will be doing”.

This person has worked with me in the past and they knew that I have a secret list of necessary software whenever I do GIS or we get a new starter here & this software is a minimum requirement for any ArcGIS user, if you have anything which I don’t mention, then please let me know, I would love to expand this list to be fully comprehensive.

  1. XTools Some tools Free or $250 for the full version                                                         This has been around since ArcGIS 3.2 and provided near ArcInfo capability without the escalated price tag. Okay, so ESRI have upped their game and now in ArcGIS 10.1 they have provided some of the…

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Create an awesome 3D map in minutes in #QGIS

Until recently, 3D has been a bit of a gimmick to the everyday GIS user, by this I mean that it has been quite time consuming to create a GOOD 3D representation (easily) until now.

I’ve used some great 3D software whilst working on survey data and also visualisation of environmental impacts, Fledermaus, ArcScene, Sketchup, 3DS Max, Solidworks, Caris to name a few… thing they all have in common though, is their weight.

If I want to create a quick and elegant image to show a client a side elevation of their project or the topography, it is quite time consuming churning through the data, then if I want to share the output….well lets not go there right now.

You could do this in mere minutes

You could do this in mere minutes

Now, before we start, I just want to remind you that you output will only be as good as your input, that is to say that, if you use good quality data, you will get a quality output. Further to this, ensuring you use the same vertical datum throughout is key, there is nothing worse that adding trees at 0m which end up floating…..

The above image was created using QGIS 2.4 and the QGIS2Threejs plugin, the data is all open source too! The DEM is the GTOPO30 (750m cells) & the basemap is openstreet map. I am sure that the OCD among you will point out that the DEM is a little too coarse for this scale and you are right, ideally a 10m or 50m cell DEM would be better but I am not creating a true life representation….I am producing a quick and simple visualisation for a client to understand their topographical issues.

Step 1: To start with, get all your software installed and make sure it all works, then set up an account with the USGS EarthExplorer, so far this has proved to be the easiest place to obtain quality for commercial & research use for free.

Step 2: Get your DEM, go to the EarthExplorer website and extract your DEM of choice. Tip here is to obtain a little more than you need, therefor if you reproject or add more data, you have a little more flexibility. Most of the formats which EarthExplorer use will work with QGIS, for this demo I used DEM format, this is ideal for the QGIS2Threejs plugin.

Selecting a DEM

Selecting a DEM

Step 3: Open up QGIS and add your DEM. I also added a raster image of the same scale to provide a reference to the viewer (otherwise it just looks like lumps and bumps).

QGIS interface with basemap and DEM

QGIS interface with basemap and DEM

At this point, don’t worry about symbolising your DEM, I have also added some points and called them WTG 1 & 2, these are not 3D points, just normal point shapefiles. Feel free to create or add your own. There is no need to display these on the map, they will be drawn by the QGIS2threejs.

Step 4: Open up QGIS2threejs

Finding the QGIS2Threejs plugin (once installed)

Finding the QGIS2Threejs plugin (once installed)

The interface

The interface

To make it easy, we will ignore the first 2 options “World” & “Control”. If you wish to reduce the extent of your model or vary the exaggeration, then this can be done under the “World” option.

Select the “DEM” option and you should find that the DEM is already selected, if not, add it here. Already the plugin has selected quite a good resolution but feel free to tweak to suit though keep in mind that higher resolutions will result in bigger and slower models. Leave the “Display type” set to map canvas image.

Step 5: Make some pretty turbines – go to the point (or if you have polygons the polygon) option.

Set your points to look cool

Set your points to look cool

Set your base height value to 0. This is the height which you want the point to sit from the surface, therefore by setting it to zero, it will sit directly on top and any height you provide the point will be from the surface (DEM). In the “Style” option, I set my point to “Cylinder” so that it would look a little like a wind turbine, though feel free to experiment!

If you also chose “Cylinder” as an option, you will now have the option to set the radius and height (from the DEM) you wish to set your point. For my map, I set the radius to 20 (where the projection system is in metres, this will mean that 20 = 20m), 20m is around the average turbine width and I then set the height to 150m, around the same height as a commercial turbine.

Step 6: Set a file path and run!

Step 7: Play with your new 3D model in your browser….

Step 8 (Optional) – Add a tilt shift effect to add that extra level of cool. Take a screengrab of your model and then save it in a quality image format (.png, .jpeg). Open tiltshift maker and upload your image…create something AMAZING!

Pretty easy eh? Using QGIS 2.4 has made a huge difference, previous attempts to write this blog over the last year has been tricky….it’s only been since the latest iteration that I would say that this is an option for the average GIS user.

This isn’t just a toy though, I have tried running 1m xyz data for a client on ArcGIS (ArcScene) & Fledermaus (DMagic) and waiting a few hours for it to process only to find it render in 5-10mins in QGIS2threejs. Although this has some way to go to be capable of being used for survey analysis and professional 3D modelling, it is hard to ignore when it is so efficient and easy to use and SHARE.

Yes, I said share….what makes this so handy is that you can zip up the file(s) which it creates and send them to someone, they can then extract the zipfile and open the html in their browser! There is also the option of hosting the model, though I am not going to go into the details of port forwarding or web hosting today..

A free 3D software for the masses which doesn’t require a MSC in geodetics, the option to share models quickly and easily?….the 3D revolution starts here!

Nick D

Web Mapping for Dummies – My personal experience

2014-06-23 19.14.16

Little baby “Kit”…that’s his name and not a DIY project

I’m back! Yes, I know, I said that a month ago but baby number 4 arrived as well as building a new intranet map system for the company I work for AND writing articles for a magazine set to take over the world – xyHt.

Building your own web mapping system has become a right of passage in the GIS world, it’s easy to buy something off the shelf like ESRI’s ArcServer but how does it really work and there is still a lot of learning to be done with cost being related to “number of cores”, “spatial database format” & my personal favourite, “server OS” (I mean, a server is a server right?!).

I went down the ESRI route and it is a phenomenal system, when it works, it is unstoppable and you have full support to answer all those crazy questions but at a huge cost, when I moved jobs to a smaller company and they wanted an internal mapping system & told them about ArcServer they laughed……in hindsight I can see why now, £100,000 for a web map system for 40 employees is a bit OTT.

As a sort of cleansing of the soul, I thought it might be useful for those of you entering into the world of building web maps to share my love/hate of web maps so far……

Mapserver or Geoserver….that is the question

Okay, lets clear this up right from the start, if you are building a web map system, you are going to be looking at open source. Sure, there are other methods like Mapbox, CartoDB, QGIS Cloud, LizMaps and others but they mostly promise a free system for web mapping but then give you (a rather meager) 50mb of space or don’t include the tools you need, this is better for the smaller business but at the end of the day you will feel cheated….for full control you really need to build it yourself from scratch.


Your first map will most probably look like this…the infamous “pink tiles”

When I first started out with this web mapping 6yrs ago, there were 2 options if you wanted to create a web map, Geoserver or Mapserver. Both work with Windows & Linux, both are well developed and both are open source, completely free and extremely develop-able with some great tools included out of the box, the problem I had was choosing one….

On paper, Geoserver and Mapserver are pretty identical, both publish geospatial data from almost any format and have huge communities who can provide help and advice. If you get really stuck, there is also commercial support for both systems. The primary difference is the choice of programming language, although both systems use a combination of languages for certain tools, Geoserver is Java based whereas Mapserver is C at its core with a lot of PHP.

Okay, so let’s roll back, already we are talking programming languages! To get on the web you need a way of getting the data into a format that can be read by the internet, to do this you need a server, for maps you need a geo/map server, something that takes the shapefiles or dxf files and gets it onto a html page which can be put on a webpage somewhere.

A simple explanation for the map server

A simple explanation for the map server

As you can see from the above diagram, the web server is the “converter” to get the data onto the internet, the data is “served” from the computer/server onto the internet. The format the data is most commonly used is Web Map Service (WMS) format.

Out of the box with both Mapserver and Geoserver you will get a server (Jetty/Tomcat/other), tools for conversion of data built in (GDAL, fTools), a LOT of projection systems (Proj4), OpenLayers (a web interface tool) and a demo map. Both systems initially install to your “LocalHost” which is a local network for your computer (it can’t be seen by anyone but you, though you will have a http address like http://localhost), to get the web server to be seen by the outside world you will need to adjust the settings in the systems server config to read the same as your outward going port connection (This may be worth checking out)

Mapserver or Geoserver….that is STILL the question….

So, going back to the Geoserver v Mapserver question, my first choice as a complete noob was Mapserver. Why? Because it sold me on its integration with QGIS, see my previous posts on how awesome QGIS is, essentially you can build your map in QGIS and then hit the “export to Mapserver” button and you have a webmap….if only….. Don’t get me wrong it is 100% better than building the map yourself but to a noob I assumed I could just use shapefiles and rasters and shunt it all to the internet with a shiny big button.



Once the Mapserver is installed and QGIS is up and running I stupidly expected to have a fully web mapping experience which would take over the world……not quite, QGIS provides you with a .map file which you then tweak and then you need to build a html page and the map tools (openlayers) around it. I may have spent my youth programming my Sinclair ZX80 & ZX81 but this was a huge step to start understanding how CSS & HTML worked and the OpenLayers (I’ll refer to it as OL from here) worked.

Tip 1: Firebug is your friend – you’ve probably seen a little picture of a bug in the top right of your screen when using Mozilla Firefox, this is a great tool for exploring the code which makes up webpages. Turn it on and waft your mouse around the page & you can see how bits of scripts work (or don’t work).

I am not afraid to say that my first map resembled something worse than a bad ZX Spectrum game, the tools didn’t work properly due to bad links, some of the layers didn’t draw up right and worse of all I felt completely out of my depth….I would post questions on the forums and it would appear that the questions I had were SO BASIC that they were mostly ignored…..

After 1 month of developing what I thought would be a new Google Earth, I put it on hold to think things over.

Don’t give up, it’s not as bad as you think

After a couple of months of chewing it over and feeling like a failure, I got back on the horse & read a couple of tutorials on the QGIS website written by the rather amazing Anita Graser. I’m not ashamed to say that I have a bit of a fan-crush on Anita, I contacted her on Twitter a few times when trying this system and she was so patient and really helped to make it easy to get going. Needless to say, after another couple of weeks of trial and error, I had a web map up and running.

Tip 2: Don’t be afraid to ask! Sites like GIS StackExchange are supported by some of the top people in the industry with some great advice. Even the dumbest questions get answered, we’ve all been there!!

Tip 3: Download and install PostGres & PostGIS – PostGIS is a spatial database system, much like the ESRI geodatabase, which is SQL based & is a more efficient and flexible way of storing spatial data. It is the most common way of storing data for web mapping & you will find it easier to work with this format in most systems.

A change is as good as a rest

Although I was having success with Mapserver, you always get that niggling thought that the grass is always greener on the other side, so decided to try Geoserver.

Much like Mapserver, Geoserver works straight out of the box on the localhost and has a nice interface for importing and managing the data.

Geoserver interface

Geoserver interface

To be honest, although the Mapserver had an easy system for making a map in QGIS, this interface made it easy to understand what was going on. I could load up layers, apply styles (in SLD format) and then use the layer preview to see what they would look like. The only downside? There is no map system integrated into it, so it can server WMS, WFS & others but you need to have a little HTML know-how to build it all into a map.

Tip 4: In Geoserver, layer groups are your friend, with these you can either create a basemap by grouping several layers together OR you can server a simple static map by using the wms direct in your browser

Tip 5: Static and Slippy…..There are 2 different types of web map, a static map which is nothing more than a picture or an interactive map which is known in the industry as a “slippy” map.

The great thing with Geoserver is that if you DON’T want to build a web map and just want to supply layers via wms to people, it is near perfect! The wms feed can be used within most GIS systems like QGIS, ArcGIS, uDIG, CadCorp and GRASS, to name a few. Again, the time and effort with this system is mostly spent designing the web map interface, calculating where tools need to sit, getting the legend to expand properly or getting the frame size just right with the company logo in the right place.

Lessons learned

The one thing I have learned from all this? Don’t be afraid to dive in, everything I learned from playing and trying to build in Mapserver, although in a different language, was similar in structure within Geoserver. The components and way the core works is essentially the same, it’s just deciding what you prefer in a system.

Once you have started to build one of these systems you begin to respect and appreciate the complexities of software like ArcGIS & QGIS but at the same time you start to understand how it all fits together. Although a stressful and humbling experience, it is one I recommend to any GIS noob.

Neo: "I know Kung-Fu" Morpheus: "SHOW ME"

Neo: “I know Kung-Fu”
Morpheus: “SHOW ME”

Why go through all the pain?

Okay, so here is where I unveil the ninja-tricks, why go through all the pain of the above? What if there was a system which had the awesome interface of Geoserver but also integrated with QGIS, automatically installed PostGIS AND provided a map interface with a plethora of useful tools….all open source with the safety net of support (at a fantastically cheap cost) if you need it?

It’s true, I kicked myself 2yrs ago when I found OPENGEO SUITE by Boundless, although it has its quirks, it takes all the pain out of everything. Sure, there is some configuration to do, some of the tools like the print and CSS styling don’t work out the box but that is what people like me are here for (contact me for installs of OpenGeo or help adding tools).

First off, this isn’t a sales pitch, although I install OpenGeo Suite systems, I do it because since I found it, I want the world to use it….you could spend hundreds of thousands of pounds on a web mapping system which has a GIS front end and a web map output. OpenGeo Suite really is as simple as creating your map in QGIS (or ArcGIS then using GeoCat bridge) and then exporting it to Geoserver, you then use their GeoExplorer to show the layers.

Standard GeoExplorer interface

Standard GeoExplorer interface

The point is?

Building your own web map system as a noob is scary and complex, lots of new terminology & methods. The developer & programmer types will bombard you with questions about your tile caching or getfeatureinfo whereas the average user will be asking whether they can upload their new NetCDF file.

Your experience will probably not be as bad as mine, things have developed FAST! Mapserver is now part of QGIS and there is a “QGIS Server” which is Mapserver based & Geoserver is now in version 2.5 – There are now so many users on Twitter & StackExchange that getting help is very easy.

My advice? Build a solution from scratch as a hobby, spend a little time every evening having a read and a play with either Geoserver or Mapserver….try and put your own map on a localhost system. When you can do that, install OpenGeo Suite and dazzle your employer!







GPS – a quick guide

Hey all, just found this on & it is a great read, hope you find it useful!

GPS Accuracy and Limitations

GPS is proven to be a very valuable tool for the purposes of Surveying and Navigation however its users must be aware of its characteristics and cautious of its limitations.

Common Factors affecting the accuracy of GPS:

  • GPS Technique employed (i.e.: Autonomous, WADGPS, DGPS, RTK, etc.)
  • Surrounding conditions (satellite visibility and multipath)
  • Number of satellites in view
  • Satellite Geometry (HDOP, GDOP, PDOP etc.)
  • Distance from Reference Receiver(s) (non-autonomous GPS ie: WADGPS, DGPS, RTK)
  • Ionospheric conditions
  • Quality of GPS receiver

Precision and Accuracy

Although precision and accuracy are often assumed to be the same thing, technically they are slightly different. Precision refers to the closeness to the mean of observations and accuracy refers to the closeness to truth.

Care must be taken particularly when using differential GPS to the accuracy of the results (closeness to truth) as reference points used can and often are inconsistent with truth.

The precision or accuracy quoted by many GPS manufacturers is often done using a statistic known as CEP (Circular Error Probable) and are usually tested under ideal conditions.

sp = standard deviation of latitude
sl = standard deviation of longitude

CEP = 0.59[sp + sl]

CEP is the radius of the circle that will contain approximately 50 percent of the horizontal position measurements reported by the GPS receiver. This also means that 50% of the positions reported by your GPS will be outside of this circle.

Another common measure of accuracy is 2DRMS (Distance Root Mean Squared).

2DRMS = 2*sqrt(sp*sp + sl*sl)

2DRMS is the 95-98% probability that the position will be within the stated 2 dimensional accuracy. The probability varies between 95-98% because the standard deviation of latitude and longitude may not always match.

Two plots are shown below (data courtesy Satloc)

Each has been created using 24 hours of data taken at 20 second intervals in the south western USA.

Figure 1:

GPS Corrected with WAAS

Figure 2:

Autonomous GPS

There are 4 techniques commonly used for GPS Navigation: Autonomous, WADGPS and RTK. Surveying applications usually require the use of RTK or Post Processing.

When used properly under ideal conditions, the CEP precisions for each method will depend on the quality of the GPS equipment in use and is approximated below:

Autonomous <10m
WADGPS 0.3-2m
RTK 0.05 – 0.5 m
Post Processed 0.02 – 0.25 m

Accuracy (closeness to truth) of differential systems is relative to the accuracy of the reference points used.

When used in less than ideal conditions, the accuracy and precision of any GPS system can be degraded significantly.

Ideal Conditions

Ideal conditions for GPS Surveying or Navigation are a clear view of the sky with no obstructions from about 5 degrees elevation and up.

Any obstructions in the area of the GPS antenna can cause a very significant reduction in accuracy. Examples of interfering obstructions include: buildings, trees, fences, cables etc. Obstructions may have the following effects thereby reducing accuracy:

  • Reduced number of satellites seen by the receiver
  • Reduced strength of satellite geometry (Dilution of Precision (DOP) values)
  • Satellite signal multipath
  • Corruption of GPS measurements

Multipath is caused by GPS signals being reflected from surfaces near the GPS antenna that can either interfere with or be mistaken for the signal that follows the straight line path from the satellite. In order to get an accurate measurement from a GPS satellite, it is necessary that the signal from the GPS satellite travels directly from the satellite to the GPS antenna. If the signal has been reflected off of another surface prior to being received at the antenna, its length will be greater than was anticipated and will result in positioning error. Multipath is difficult to detect and sometimes hard to avoid.

Figure 3

Other Sources of Error in GPS

  • Signal Delay caused by the Ionosphere
  • Signal Delay caused by the Troposphere
  • Orbit Errors (GPS satellite position inaccuracy)
  • Receiver Noise

Common GPS Surveying and Navigation Techniques and Associated Errors

Autonomous or Stand Alone

The method involves using a GPS on its own with no additional correction information other that what is broadcast by the GPS system. Prior to May 2, 2000 accuracies obtained using this method weren’t usually much better than 100m due to a US Department of Defense induced error called Selective Availability (SA). On May 2 SA was turned off and now accuracies are usually better than 10m.

Autonomous receivers will attempt to correct the Ionospheric and Tropospheric errors bases on mathematical models which are very limited in their accuracy. They have no way of correcting for orbit errors, multipath or receiver noise.

Wide Area Differential GPS (WADGPS)

Examples of Systems that use WADGPS include:

These systems receive an additional satellite signal that contains more accurate information about GPS Ionosphere and Orbit errors allowing the GPS receiver to determine a more accurate position. These systems have no way of correcting for multipath or receiver noise. Accuracies of WADGPS are often better than 2-3 meters. Although multipath can cause very large errors as is the case in the Autonomous positioning. A solar maximum of a an 11 year solar cycle occurred near the year 2000 which can also have dramatic and unpredictable effects on the accuracy of WADGPS systems.

Real-Time Kinematic (RTK)

Many GPS receiver manufacturers provide a system that employs a technique known as RTK. RTK implements the use of much more complex GPS data processing than other techniques, although RTK can eliminate many errors characteristic of other systems. RTK has additional limitations.

When using RTK, a reference receiver (Base) must be placed on a known reference point. This reference receiver then transmits measurement or correction information over a radio link to the roving receiver (Remote) that will be used for positioning or navigation.

This technique can result in accuracies as good as 0.05 m – 0.10 m if used properly and in ideal conditions.

The limitations of an RTK system include the following:

• Initialization – The receiver must be initialized in good GPS conditions for up to 15 minutes before achieving sub-meter accuracy. If the receiver sees less than 4 satellites at any given time after being initialized, the receiver must re-initialize before again achieving sub-meter accuracy.

• Baseline Length – As the distance between the Base and Remote receivers grows larger, the errors observed between the GPS receivers becomes less and less common degrading accuracy at the remote. Good accuracies can normally be achieved with baselines (line between base and remote) in the order of 10 – 15 km. Baseline lengths can be reduced considerably when strong ionospheric conditions exist.

• Radio Transmission – The base and remote must maintain communications at all times in order to maintain good accuracy. Terrain, distance and interference all have effects on the distance in which the base and remote are able to maintain communications.

• Visibility and Multipath – Usually at least 5 satellites must be available in order to achieve good results. Although less susceptible to multipath after initialization compared to other techniques, RTK results can seriously be degraded by obstructions such as trees, fences and buildings.

• Accuracy of Reference Point – The absolute accuracy of the position reported by the Remote receiver is only as accurate in an absolute sense as is the position of the base station coordinates.

Post Processing

This technique of GPS is used mostly used for Surveying and is not used for navigation. It is similar to RTK in that a base station must be placed at a known reference point and a rover is used for gathering new positions. Instead of obtaining accurate results in real-time, accurate coordinates are generated by taking data stored from the receivers and processing them using special software on a computer. Extremely accurate results in the order of a few centimeters can be obtained if done properly and the conditions are good but post-processing is subject to many of the same limitations as RTK.


All GPS navigation and surveying techniques have limitations that may not permit desired accuracies in a given environment. The cause for poor accuracy is not always obvious but is usually attributable to one of the following source of error:

  • Multipath / signal corruption
  • Low number of satellites / poor satellite geometry
  • Erratic Ionospheric activity

These errors can lead to position errors as large as several of meters or more.

How do smartphones know where North is?…..

Conversation on Twitter, where someone pulls a statement on you and you KNOW it’s true but at the same time your brain is wondering why, you hadn’t thought about it before….yesterday was one of those days. I made a statement about whether the compass was dead and whether this is the rise of the GPS (mobile) compass, taken from an article written by the Ordnance Survey- Here

Obviously someone pointed out that GPS needs at least 3 points to calculate direction (an m, movement) value….couldn’t argue, completely right but something was telling me that my phone isn’t that stupid to tell me where North is with only 2 points.

I’m not stupid (well……)

A typical smart phone has three magnetic field sensors, fixed perpendicular to each other, which are used to work out the local direction of Magnetic North.

In addition, they have three accelerometers which sense gravity to give tilt information and to help work out which way is down.

Image courtesy of British Geological Survey

Further to this, my phone is even more clever than I realised, when it can’t get a GPS fix it will calculate where I am using its angle of approach to the cell tower, how long it takes the signal to travel to multiple tower & the strength of your signal when it reaches the towers. Since obstacles like trees and buildings can affect how long it takes your signal to travel to a tower, this method is often less accurate than a GPS measurement but it is still pretty clever!


Unfortunately mobile GPS is not that accurate compared to a dedicated GPS unit, this is usually due to;

Chipsets. Dedicated GPS, and Smart Phones use a chipset to actually perform the GPS calculation. The accuracy and capabilities vary. Some chipset’s are optimized for urban canyons and are particularly suitable for smart phones (SIRFstar III). Others can track a greater number of satellites concurrently and feature SBAS augmentation. Some cell phone chipsets may outperform those used in dedicated devices. Even some professional grade GPS (Nomad 900G) units ship with less accurate consumer grade chipsets. Antenna. The placement and quality of the antenna effects GPS accuracy. Some dedicated GPS units have external antenna’s instead of the internal antenna typical in smartphones. They may also feature an antenna socket to allow an aftermarket antenna to be added.

How bad?

The difference between smartphone GPS and dedicated GPS is shortening, in 2009 the difference was around 20m, today a mobile GPS will get a fix within 5m of a point, or if ESRI are to be believed, 3m. In this blog ESRI discusses mobile GPS in phones & tablets v dedicated units.

Nick D


Back before you’d ever realised I’d gone….

Those of you who read my blog may have realised that I’ve not written for a while, in fact for a month or so….without getting too personal, a couple of months ago I dislocated my left leg. Being a bit of a bloke, I popped it back in and thought it was okay, went up A&E the next day to find that they wanted to put me in a wheelchair for the rest of my life. Yup, it’s been a little hectic for the last few months but be rest assured, I am walking and (more importantly) back to doing the blog!

Goodbye to #GISChat

Whilst I have been pre-occupied, a lovely lady called Emily has started up a Wednesday Twitter chat called #GeoTribe. Held every Wednesday at 8pm GMT, it is a great chance to catch up on what people are talking about and to meet like-minded geopeople.

Unfortunately, it does mean the end of #GISchat as the two coincide. I am sure people won’t be too devastated and I look forward to talking to you all on the #geotribe.

Hello to Geoserver…

Those of you who I’ve met at the recent round of conferences will know that I’ve got into Geoserver and selling Boundless’ OpenGeo solution. Now, lets get this straight, I’m not selling the software but selling the skills to install it, get the plugins (like the print module and QGIS) working & also provide a little customisation at an extremely CHEAP price….only problem, you’ll have to put up with me chatting about Geoserver A LOT more….

Okay, I’ve rattled on enough and said nothing GIS, so I will leave you with this; Google Earth displays in WGS84 but calculates in Web Mercator……