Category Archives: SIX/GIS

INTRODUCTION TO GIS (V)

Time has passed since my last post on GIS, sorry for the waiting. But here we are on track again, this time with an essential one… how to add the points from your GPS to your map. Remember you can download all tutorials from my Academia.edu page. Enjoy!

Vector points:

So now that we have our raster map of the region we want to study it is time to add some sites to it. In order to do this we will create a vector map from a list of points in a spreadsheet.

1.- Once you have a list of points you can organise them in a spreadsheet file like this:

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The most important things to take into account are the order to the fields, which is not mandatory but strongly suggested, and that in order to avoid issues it is better to avoid spaces at least on the headers. This is a simplified version from my Ph.D. research, and it gives you the general structure. We need only three main fields, the two coordinates and the CAT, which is just a field from 1 to n, like the index on a database. Names, types of sites and many other fields can be added after this. It is up to you include as many as you want, but for the lesson of the day we will use this simplified version.

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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.

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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.

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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.

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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.

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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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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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Introduction to GIS (II):

Maps and Software. Creating a Location:

Despite what you might think, the first thing you need to start working with maps is not a computer, a GIS program or some GeoTiffs. The first thing you need is a research question. If you do not have this, the rest is worthless, because your research question is going to condition what kind of analysis you will carry out and how. So, before starting with these lessons I would suggest you to think about your research area and what would you like to know about it. You can be interested on exploring which areas are under the visual control of your site, or the most efficient route between a workshop area and the place where raw material were collected. Every example in these posts can be tried with any map, but if you do not have any specific research area or just want to give GIS a try you can use the maps of the area of Korinth that I will use as an example.

The software I am going to use is GRASS. This is an open source program supported by a community of developers around the world, which include some of the big names in Landscape analysis such as Michael Barton, from Arizona State University. You can download it for free here. Everybody has his/her favourite program, and usually causes intense debates around beers on which is better and why the rest are useless. GRASS is my favourite one for two reasons: first, it is one of the most reliable GIS programs and it is constantly updated by a community of developers. Secondly, it is completely free. Many other programs are very expensive and they do not offer, in my opinion, any analytical advantage over GRASS. It is true that it is perhaps less user-friendly than other programs, but you will see that once you get used to it, GRASS is very straightforward.

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Introduction to GIS (I):

GIS (Geographic Information System) technology is an essential tool for any archaeologist; nobody would argue that. But in my experience as a scholar and also teaching introductory courses to GIS, I have realised that, in many occasions, archaeologists are not totally able to explain what a GIS is. This is caused in my opinion by something I call the tyranny of the technology. Let me explain.

Roger Tomlinson, the father of GIS, was the first person to use this terminology, in a paper published in 1968 entitled A Geographic Information System for Regional Planning. This work was the result of the development of the first true operational GIS, the Canada Geographic Information System in 1960, because, and this is the key factor, computers are essential for GIS.

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Shaded relief of the Corinthia

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