Maps, models, and mechanisms

There seem to be more and more habitat models being published. The idea is usually to take a set of geo-referenced records of a particular plant or animal in an area; find a statistical association with a gridded set of environmental variables; then extrapolate the likelihood of that species occurring in all of the grid squares in the study area. The result can be called a habitat suitability index, or a similar term, and can be used to help inform management policies, protected area planning, and forecasting the effects of future change scenarios.

I guess that the increasing number of such modelling efforts is in part due to the very welcome increase in environmental data – factors such as rainfall, elevation, protected areas, towns, rivers, land cover and vegetation indices are readily available for pretty much anywhere in the world. Added to this is the availability of free software, GIS and statistical, as well as far greater access to computers and knowledge of relevant techniques.

So I was interested and pleased to come across this piece from Colin Beale at York University: it is not only a constructive critique of a particular research paper that used such techniques, but it widens the discussion to the nature of explanations in biology:


Google Earth installer

If you want the full, self-contained installer for Google Earth, rather than the default web installer, here’s the answer.

In short, there’s a discreet “Advanced setup” option on the Google Earth download page.

(For GE v6)

Juba – boom town

From a remote sensing perspective, being in Juba in 2007 was slightly frustrating: although Google Earth covered South Sudan’s capital with a hi-res QuickBird image, it remained stubbornly in 2003. Sometime during 2010 however it was updated, to a 2009 GeoEye image. Comparing the two shows just how dramatic the urban sprawl has been.

click for full size

Google Earth image from 5 Dec 2003 (QuickBird)

Google Earth image from 16 March 2009 (GeoEye)

In fact, most of the growth in this patch has been post-2007: when I left then, the situation was much as shown on the 2003 picture, so most of the building has taken place in just two years.

Note that the US consular swimming pool, in the leafy compound  south-centre of the image, is visible in both, a reflection perhaps of American intrests in the area!

Update, 11 February 2011:

Google has just added some recent imagery from GeoEye to its Earth servers: very recent, in fact, from 30th January 2011, covering several parts of Southern Sudan. In Juba, the clearest change since the 2009 imagery (above) is the colour of the roads. Red has turned to grey: the seemingly endless construction programme has resulted in a lot of paved surfaces. It’s not just the airport road, but all of the main routes around town. A lot less dust in the air, I imagine.

Google Earth image from 30 January 2011 (GeoEye)


Adding an arrow to an OSM map

Should be easier than this I reckon, but here’s how to do it and it’s hardly hard:

Quantum GIS – using a USB GPS device

In reposnse to an email question on the TZGISUG list “I am trying to get my Garmin GPS etrex Vista HCx to communicate with Quantum GIS through my pc with window 7. I will appreciate your comments and tips on how to get this working. Mtemi H. Miya M.Sc.”:

First of all, check the following:

  • Your GPS communicates with your PC normally, e.g. using MapSource, Google Earth or GPS Utility.
  • You have the “GPS Tools” plug-in for Q-GIS.
  • You have GPS Babel installed (I THINK this happens as part of the Q-GIS installation).

In GPS Tools, the default GPS device is a Garmin serial connection; so if you have a serial cable and connector, you’re in luck. However, most of us now have USB cables, which complicates matters. You need to define a new GPS device in GPS Tools, as follows:

  • GPS Device > Edit devices > New device >
  • Name = Garmin USB (or whatever name you want)
  • In the following six command lines, paste in the following (user-friendly, eh?!)
  1. %babel -w -i garmin -f usb: -o gpx -F %out
  2. %babel -w -i gpx -f %in -o garmin -F usb:
  3. %babel -r -i garmin -f usb: -o gpx -F %out
  4. %babel -r -i gpx -f %in -o garmin -F usb:
  5. %babel -t -i garmin -f usb: -o gpx -F %out
  6. %babel -t -i gpx -f %in -o garmin -F usb:
  • Update device > Close

Then still in GPS Tools:

  • Set the GPS Device to the new one you’ve just made (in this case, Garmin USB)
  • Specify the new filename and new layer name. This is where I got stuck for ages! If there is a space in the file name or path name, you’ll get an error message ? This isn’t documented anywhere. So, avoid the desktop, my documents, etc, and check the full path has no spaces.
  • Finally click OK. And hope for the best!

How tropical is the world?

“Rivers are the major conduits for the transfer of water, particulates and dissolved material from the land to the ocean and more than 50% of this flux enters the global ocean in the tropics” (*)

This sentence made me wonder, incidentally, what proportion of the world’s surface area is contained between the two tropics. A friend pointed me to two websites that answer the question.

First, someone who had wondered just the same thing:

And a list of formulae relating to spheres:

The answer is surprisingly simple: the surface area of a sphere (radius ‘r’) that lies in between two parallel planes (‘h’ apart) = 2*pi*r*h !!

In the case of the tropics, h = 2*sin(23.5)*r … so the answer to the tropical surface area is approx. 0.398, or 39.8%, or about two fifths.

Next step (for the GIS) is to calculate the proportion of the land surface area that is tropical…

(*) PETER J HEDLEY, MICHAEL I BIRD, RUTH A J ROBINSON. (2010) Evolution of the Irrawaddy delta region since 1850. Geographical Journal 176:2, 138-149

Using OS MasterMap in ArcGIS

Quick links for reference:

Regarding different transformation methods between OSGB1936 and WGS1984:

For conversion of MasterMap GML to various formats, including shapefiles, using InterpOSe:

Incidentally, a pretty useful description of transformations:

Links from Edina Digimap: