Introduction to GIS (IV):

Reprojecting to UTM:

So now we have a map that covers most of our working area, but the coordinates are in LatLong. The problem is that this is a sexagesimal system, and thus not very good for some of the operations we want to carry out. So, before we can finish our base map we need to covert it to UTM, a decimal system. It is a simple operation, once you get used to it, but in my experience it takes a couple of attempts before the reprojections are done with confidence. Leave any questions if some of the steps are not clear.

1.- Open your patched map. In Settings ~> Region ~> Set region we will set the region of the map. A region, in simple words, represents the boundaries of your maps, its mathematical limits. It is important to always know where the boundaries of your map are, since any calculation will end at the limits of the region, and many times those are not the same that the ones in the map. In the menu select Set region to match raster map and choose your patched map in the drop list. Then press Close.


2.- By doing this we are able now to create a vector map that will be our “template” to be able to reproject the map. In Vector ~> Generate area for current region. In the Required tag of the menu create a Name for the map (remember to avoid spaces). In Optional be sure the option of As area is selected. Run and Close. If you check this new map you will see that it is just a grey box exactly matching the limits of your map. That is what we need. We need to close GRASS and open it again.

3.- In order to wok with Utm we need to create a new location for our map. It is the same method we learnt in lesson one, but this time we need to Select coordinate system parameters from a list. In the next window, look for UTM in Choose Projection.


4.- What we have done with all those steps was to create one location that fits our needs, rather than extracting the information from a file. But we still need to “translate” our LatLong map to the UTM location. For this we access the permanent mapset and we will use the vector map we have created before to establish the boundaries of our map.

5.- In Vector ~> Develop raster map ~> Reproject vector map from different GRASS location. In Required select the location in which you have made the vector map. In Source select first Mapset and then name of input map. Press Run.



6.- Now you have reprojected the vector map to UTM. Perhaps you have notice that it is not a rectangle but a trapezoid. That is actually what you are looking for, because it indicates that the map was reprojected properly.

7.- Set the region as in step 1, but this time Set region to match vector map.


8.- We are almost there. The last step is to reproject the raster from the Latlong location to the UTM location. To do this, go Raster ~> Develop raster map ~> Reproject raster map from different GRASS location. The Required and Source steps are the same that those for the vector. The difference in a raster is that we need to choose a method of interpolation (lanczos_f) and indicate the resolution of output raster map (30). In simple words the method is the way the software calculates the value of the cells of the new map. Other methods such as nearest or bicubic are faster, but they affect the results of the output map. Lanczos_f allows us to do the task without loosing information. The reason for selecting 30 in the resolution is because it is the same that our source map (ignore bicubic on the picture, choose lanczos_f option).

And that is all! I know this task can be a little bit confusing, but it is essentiall for your work on maps. Now you have your map in UTM units, ready to work and to be used on some operations we will exploring soon. If you have any doubt please leave a comment!

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