Tag Archives: palaeogeography

Georeferencing a Palaeogeographic Map

There a a number of resources on the web and in the literature when it comes to palaeogeographic reconstructions. Two of the most commonly used sites for images are Chris Scotese’s PALEOMAP project and Ron Blakey’s home page. While the images that you can find on them are great, the format the images are in are a bit of a problem when it comes to loading them into a GIS program, as they aren’t in a typical GIS format. For example, the common GeoTIFF format that many GIS images are distributed as contains information about the image’s location, allowing it to be easily opened in GIS program. This makes it easier when trying to combine multiple georeferenced files, as the program will automatically place them in the proper location (and projection) relative to one another.  For my research, this is a very useful tool as it allows me to quickly place and update fossil localities on a map, without having to do everything by hand. Luckily, there are several easy ways to georeference an image.

The first method involves using a cross-platform GIS program called Quantum GIS. Once you have the program installed, open it up and the first thing you will do is enable the Georeferencer plugin. To do this, go to  Plugins > Manage Plugins on the top menu bar. Scroll down and select the Georeferencer plugin, and hit OK. Now, go to Plugins > Georeferencer > Georeferencer to open up the plugin. You will bepresented with two windows. Go to the window titled Georeferencer and navigate to the location of your image that you want to georeference and select it. Once you’ve done this, your image will pop up in the other window. In this window, you will select points on the map for which you know the location, and these points will allow the GIS program to position your image accordingly. To select a point, choose the button with the three red dots and then click on the image. It will ask for the X (typically longitude) and Y (typically latitude) of the point, which you need to enter. You can use the zoom options (top left) to zoom in and out to be more precise when choosing points as well. Once you’re done, you can hit Create to save or Create and Load Layer to save and load it in the program. Your original image should be unaltered, but there will now be a world file with a similar name (but slightly different ending) that contains the coordinates for the image, which the GIS can then use.

While the first method may seem easiest, as it just involves clicking pictures, I prefer a second method. I have installed a command line program (well really, a library and associated utilities) called the Geospatial Data Abstraction Library or GDAL for short. Here again, there are packages for all platforms available for download. Within GDAL, there is a utility called gdal_translate which allows you to georeference an image. To try it out once you have GDAL installed, fire up a terminal (or console, or whatever you like to call your command-line interface). From here, a single line of text can georeference your image. For example, if I have a full world map that is 375 pixels high and 750 pixels wide (as are all of Blakey’s palaeogeographic maps), and I know that the corners of the map are at -180 and 180 degrees longitude and -90 and 90 degrees latitude (as are Blakey’s), I can use the line:

gdal_translate -of GTiff -gcp 0 0 -180 90  -gcp 0 375 -180 -90  -gcp 750 0 180 90  -gcp 750 375 180 -90 90mya.jpg 90.tif

To break down the line a bit, gdal_translate calls the function, -of GTiff tells it I want a GeoTIFF as the output format, and the -gcp 0 0 -180 90 etc. each refer to a corner of the image and it’s longitude (or X) and latitude (or Y) (ie the top left corner or 0 0 is at -180 degrees longitude and 90 degrees latitude). Finally, the last two parts refer to the original image name (90mya.jpg) and output image name (90.tif). While this may seem a little more complex at first, it is far more precise than trying to click exact points on maps, and is much faster to use, especially with multiple images of the same size and position.

Either way, in the end you should have a much more useful image that you can do lots of other things which, like changing to a new projection, which I’ll talk about a bit more in a little while.

Palaeogeography in Google Earth

google_paleoearthWhile playing with some palaeogeographic maps from Ron Blakey’s website recently I was trying to come up with some way of manipulating the maps in a fun, and really visual, manner. I found somewhere about instructions on overlaying images on the globe in Google Earth, probably meant more as a way to place smaller regional maps on the globe. However, I realised that you can overlay a new image across the entire globe as well. What this means is that I could take a palaeogeographic reconstruction and manipulate it in 3D much as I would the modern geography in Google Earth. The picture on the left gives you an idea of what it looks like.

The whole process is pretty simple too. Download an image for whichever time period you like (make sure the map is straight up lat/long, and not a Mollweide projection). Then, open up Google Earth and select Add > Image Overlay and select the image you just downloaded. You probably need to tweak the placement a bit though, so under the Location tab in the Image Overlay window, be sure that you set the boundaries to 90 degrees North and South and 180 degrees East and West, and hit OK. Once you’ve done that, you can spin the globe around and take a look at how the Earth once appeared. One neat thing too is that you can adjust the transparency of the image overlay by using the slider just above the Layers on the left hand side. Have fun!

Map projections and gdalwarp

I recently was working with some maps, trying to create some pretty pictures for my thesis using some of the maps by Ron Blakey. However I was too lazy to plot points on the maps by hand and I wanted to georeference the maps. I found a hand tool with QGIS that was easy enough to use, but wasn’t as precise (or as quick) as another command-line tool I found called gdal_translate. GDAL (Geospatial Data Abstraction Library) and the associated OGR comprise a number of tools and libraries that are very useful for both raster and vector geographic image manipulation. Using gdal_translate was easiest with a straight up lat/long projection, as I could just give the geographic coordinates for each of the corners and it would spit out a GeoTIFF. Thus, I was able to quickly take some of the rectangular maps and georeference and have something that I could use in QGIS. While I was playing around gdal_translate, I found another tool called gdalwarp, which has been giving me a bit more of a headache. Theoretically, gdalwarp should let me transform some of these rectangular maps into different, possibly ‘prettier’ looking maps. So far though, all I’ve managed is to make some seriously un-pretty images. I’ll post an update in a little while giving some quick instructions once I figure out how to make it all work.