Introduction to GIS (III):

Importing and Patching Maps:

In the last lesson we learnt how to import our first map in GRASS. However, as it happens many times, our working area needs of more than one map. In this lesson we will import several new DEM maps to our location and we will learn to patch them all together in a single, larger map. I have used several .tif files that I have uploaded here. Remember to change the extension to .rar (follow the instructions of lesson II).

1.- In the Layer Manager window, select File ~> Import raster data ~> Common formats import.

2.- In the menu, select File if you need just one map, or Directory if you need more. If you choose the second option be sure the Format selected is GeoTiff and the Extension .tif. Be sure all the boxes on the left side of your map names are selected. Finally, select Extend region extents based on new dataset. Press Run. When finished, press Close.

It is time now to introduce a very important concept: Region. In the previous lesson we decided to set up the region based on the DEM file. Imagine a Region is like the boundaries of your location. It has an N, S, E, and W limit, right now just the ones of the map we used in lesson 2. But if we add new maps without changing that region, those boundaries of our location, we will just pile up one on top of the other. That is why we select Extend region extents based on new dataset, so the new maps will push those boundaries to fit the new information we are adding.

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3.- Now you should be able to see the maps you have added. If you include the map we used in lesson 2 you will have a rather decent portion of central Greece and northern Peloponnese in your display. But this is not yet a single map but just all the ones we have imported shown together. In order to patch different maps into a single one go to Raster ~> Overlay rasters ~> Patch raster maps.

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4.- In Required select the maps to be added and a new name for your map. When adding several maps you need to press the arrow on the right to select them one by one, so the software will create a large string of names in that text box. It is a little bit counterintuitive, but just go one by one carefully and you should not have any problem. When ready press Run.

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Note the long string of names in the [multiple] section.

And done. From several maps now we have a single one that represents our working area. Change the colours as we have seen in lesson 2 and try to play a little bit on the display window exploring the options and zooming in and out. As you can see in the bottom left of the screen, the map is expressed in latitude and longitude, which is a sexagesimal system and more complicated to operate with. So next lesson we will learn how to reproject your map from latlong to UTM.

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DEM map of the Peloponnese

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