NEW SKILLS Begin to learn how to add data in QGIS. Exploration of some of the vector and raster analysis capabilities of QGIS

Transcription

1 Lab 3 VECTOR AND RASTER MODELING Last modified 7 May 2014 NEW SKILLS Begin to learn how to add data in QGIS. Exploration of some of the vector and raster analysis capabilities of QGIS 1. In this exercise we will learn how to add a field to a vector database and insert values. This operation can be performed directly on a shapefile. Add the vector layer allotment.shp from the BrownsPond folder (see Lab 1 Problem 1). Click Cancel when asked for the projection. In Windows, create a new folder called Lab3 in your working folder. In QGIS, right click on the shapefile, choose Save as, and save a copy of the shapefile in this folder call it allotment1. Add this shapefile to the project and remove allotment. Open the attribute table of the allotment1 shapefile. Click the toggle editing mode button on the bottom (button furthest to the left the symbol is a pencil). Now click the new column button (should be the 2 nd button from the right confirm by moving the cursor over the buttons). Fill in OWNER in the Name box, change the Type to Text (string), and set the Width to 25. Click OK to add the new column. You re still in editing mode (until you click the toggle button again), so you can doubleclick on the NULL OWNER values to change them. Fill in the following data: Allotment 1: Wilson; Allotment 2: Peters; Allotment 3: Johnson. Click the Toggle Editing Mode icon again to stop editing and save your file when prompted. Now right click on the allotment1 layer in the Layers window and select Properties. Under the Labels tab, click the Label this layer with check box and select OWNER. Click OK. The labels should be displayed. You can play with the size and font if you wish. 2. In this exercise we will learn how to import point data from a table into a GRASS mapset or a shapefile. We will do this using GRASS tools. Start a new project and create a new mapset Lab3 in the BrownsPondGR location. (NOTE: if you can t access the location then you must navigate to the folder where this location was created). Add the buildingsgr vector layer from the BrownsPondData mapset. The buildingsgr layer does not include any stores. We will create a text file with store locations and import this into GRASS as a new map (we could add the stores to the buildings layer using the Editor as in Problem 1). The text file will look like this: X,Y,CODE,CATEGORY 2530,3550,7,Store 1075,5175,7,Store 750,5650,7,Store There are two ways to create this file. The first is to just use a text editor such as Notepad. However, if you have a large file, typically in the form of an Excel spreadsheet, you can save this file as a csv (comma separated variable) file. If you wish, create a file in Excel (or any other spreadsheet application) whose columns have the data shown, and save it as a csv file. Whether you use a text editor or Excel, save the file as Stores.csv in the Lab3 filder.

2 Now open GRASS Tools. This time we will use the Modules Tree. Select File Management ->Import into GRASS-> Import vector into GRASS -> Import vector into GRASS -> v.in.ascii Import text file. In the window that opens, for the Text file, click on the tree dots, navigate to the Lab3 folder, and select Stores.csv. Click Open. Type StoresGR as the Name for the output vector map. Leave the Input file format as Simple x,y[,z] list. In the Separator window, delete the and enter a, (a comma). In the Number of rows to be skipped, delete the 0 and enter 1 (we will skip the header row). Leave the other selections as they are and click on Run. Then click View output. You should see the three stores. If you open the Stores attribute table, you will see that GRASS has changed the headers. Now we will export the Stores layer to a shapefile. Open GRASS Tools. Again we will use the Modules Tree. Select File Management ->Export from GRASS-> Export vector from GRASS -> v.out.ogr. For the Name of input vector map, select StoresGR. For the OGR format, select ESRI_Shapefile. For the Name, navigate to the Lab3 folder and type Stores.shp. Click on the three dots to make sure it is going into the correct folder. Leave the box checked, and click Run. The next obvious step would be to join the Stores layers to the Buildings layers. We will take that up in a subsequent lab. Map Analysis using Raster and Vector Layers The village of Brown s Pond wants to create a map of suitable sites for a new fire tower. There is an existing fire tower at the fire station. The list of criteria for suitable sites is: 1. At least 4 kilometers from the Fire Department. 2. At an elevation of at least 300 m. 3. On open ground. GRASS First we will carry out this operation in GRASS. Start a new project (you don t need to save the old one). 3. We start by creating a new vector layer showing everywhere within 4000 meters of the fire department. To do this we will create via a very simple digitizing process a new vector layer containing only the fire department (there is another way to do it that we will see in Problem 5). Click Plugins -> GRASS Tools -> Open mapset and open the mapset Lab3 in the BrownsPondGRASS location. Add the buildingsgr vector layer from the BrownsPondGR location, open the attribute table, click the Select by Expression button, and select Category = Fire Dept (see Lab 1 Problem 9 for review). Now we can see the location of the fire department. Close the attribute table Click on Plugins -> GRASS -> Create New GRASS Vector, or click on the Create New Vector Layer icon. Name the vector layer firedeptgr. In the GRASS Edit window, for Mode select Manual entry and click the New Point icon (the three points), position the cursor over the fire department, and click to create the new point. Close the two windows that are open (GRASS Attributes and GRASS Edit).

3 We now have our fire department layer. Click Cancel when asked (possibly more than once) about coordinates. You can uncheck buildingsgr in the Layers window to confirm that it is there. You may also get a topo_node and a topt_point, but for purposes of this lab we can ignore these. Now we will create a 4000m buffer around the fire department. Under Project -> Project properties (General tab), make sure your layer units are in meters. Click on plugins -> GRASS -> Open GRASS tools and type v.buffer in the Modules list filter. Click on it. Select the firedeptgr layer, enter 4000 for the buffer distance in map units and fire4000gr for the name of the new layer. Click Run. Add the new layer to your project (as usual, you can do this directly by clicking View output). 4. Next we will create a vector layer identifying locations above 300 meters in elevation. We do this by first using the map calculator to create a raster layer of elevations over 300m and then converting this to a vector layer. Add the GRASS raster layer elevgr from the BrownsPondGRData mapset. Open the GRASS tool r.mapcalculator and, using the same procedure as you did in Lab 1 Problem 10 (entering elevgr as A and typing A >= 300 as the formula) to find the locations in Field 5 with yield greater than 7500, create a raster file elev300gr consisting of locations with elevgr >= 300. Add the layer. (NOTE: in my application of this step in Version 2.2, I got repeated notifications of warning messages. After clicking on the Close box of these messages repeatedly, the operation worked as expected). Add the raster layer elev300gr from the Lab3 mapset. Use the conversion tool r.to.vect.area (find it either using the filter or looking in the Modules Tree under File management -> Map type conversion -> Covert a raster to vector within GRASS) to convert the raster elev300gr to a vector layer named elev300vecgr. Click View output to add the vector layer. Use the Identify features tool to determine whether value = 0 or value = 1 corresponds to the locations over 300 m. To identify the feature values in a given layer, click on that layer in the Layers window first. 5. Next we will create a vector layer of open land. Instead of digitizing as we did in Exercise 3 we will use the function v.extract, which selects a feature. Add the GRASS vector layer vegevectorgr from the mapset BrownsPondGRData. Open the GRASS tool v.extract.where. You can do this using the filter, but at this point it is also instructive to see if you can do it by exploring the Modules Tree. Choose vegevectorgr as the Input vector map and type TYPE = Open in the WHERE box (NOTE: this is case sensitive, and you need to include the quotation marks). Type vegeopengr as the Name of the output vector map. Click Run. Click View output. If you have done this correctly, you will see a new layer with open land. You can use the Identify Features tool to check your work. 6. Now we have 3 vector layers ready: fire4000gr, elev300gr and vegeopengr. We will use two operations of Vector overlay to create the desired map. Look over the three criteria again. First we will get rid of everything less than 4000 m from the fire station from the layer showing areas above 300 m. Choose the GRASS tool v.overlay.not Vector subtraction (note that if the Modules Tree tab is no longer visible you can click the left arrow button in the upper right corner). For the first vector layer choose elev300vec and for the second choose fire4000. Name the layer el300fire4000gr and click Run.

4 Add the vector layer el300fire4000gr. Notice that you have subtracted the area of the layer f4000. Open the attribute table and use the Identify tool to determine the value of the sites over 300m. If your map is getting too complicated, uncheck some of the layers to view it more easily. Now we will overlay the open land layer. Choose the GRASS tool v.overlay.and Vector intersection. For the first layer choose el300fire4000gr, and for the second choose vegeopengr. Name the layer elevfirevegegr. Click Run. Add the elevfirevegegr layergr. Use the Identify features tool to determine how to identify the locations that correspond to elev >= 300 (Hint= check the a_cat and the b_cat and figure out what they signify). Run the GRASS tool v.extract.where Select features by attributes on the layer elevfirevege and use this selection (Answer at the end of the lab). Name the output map towersitesgr. Add the towersites layer. If you have done this correctly it should look like the map shown here. IMPORTANT You have seen how the GRASS vector modules can be used to perform vector operations. However, when going through these instructions for the first time, it is almost impossible to avoid focusing so much on each individual step that you miss the big picture. Go back through each problem, look at the operation you did and the results of that operation, and see for yourself how you used the GIS to solve the problem you were given. QGIS 7. Now we will do this project in QGIS rather than using the GRASS tools. We start by creating a new shapefile showing everywhere within 4000 meters of the fire department. Start a new project.

5 Add the buildings shapefile from the BrownsPond folder (as always with Brown s Pond data, click Cancel when asked for a projection). Open the attribute table, click on the search, and select Category = FireDept. Prepare to create a 4km buffer around the fire department by making sure your map unit is meters go to Project -> Project Properties, General tab and choose Meters under Camvas units. Confirm that you have the ftools Plugin fetched and active. You can check this via Plugins -> Manage Plugins. If you see this (note the f in ftools), make sure it is checked.. If you don t see this plugin, obtain it using Plugins -> Fetch Python Plugins; if it s there but the box next to it isn t checked, check the box. Go to Vector -> Geoprocessing Tools -> Buffer(s). Keep buildings as the Input vector layer. Make sure the Use only selected features box is checked. Enter 4000 in the buffer distance box. For the Output shapefile, Browse to your Lab3 folder and call the new buffer shapefile fire4000. Leave all other settings as the defaults. Click OK Navegate to your Lab3 folder and add the new shapefile to the project. 8. Next we will create a shapefile of locations above 300 meters in elevation. Add the elev.asc raster from the BrownsPond folder Select Raster -> Raster calculator. Make sure elev@1 is listed in the Raster bands window. Click on the 3 dots next to the output layer, navigate to your Lab3 folder, and enter elev300, and click Save. Double click elec@1 to bring it into the Raster calculator expression window, click on the >= button, and type 300. Click OK. Select Plugins -> Manage plugins and make sure GdalTools is checked. Select Raster -> Conversion -> Polygonalize (raster to vector). Choose your elev300 raster. Click Select next to the Output window, navegate to your Lab3 folder, and save as a shapefile elev300.shp Click OK. Add the shapefile to your project, 9. Next we will create a shapefile of open land Add the vegevector shapefile from the BrownsPond folder. Open the Attribute Table. Use the selection icon to select the field TYPE = Open. Right click on the vegevector shapefile and choose Save selection as. Save the new shapefile as vegeopen in the Lab3 folder. Add it to your project. 10. Now we have 3 shapefiles ready: fire4000, elev300 and vegeopen. We will use two geoprocessing tools to create the desired map. Select Vector -> Geoprocessing Tools -> Difference. Select Input vector layer = elev300, Difference layer = fire4000, browse to the Lab3 folder and save the output shapefile as el300fire4000.shp. Save the output file as distelev in your Lab3 folder. Click Add result to canvas. Confirm that this includes all land further than 4000 m from the fire department, classified as above 300m or below 300m. Select Vector -> Geoprocessing Tools -> Intersect. Input vector layer = distelev, Intersect layer = vegeopen, Output shapefile = distelvege in the Lab3 folder. Again use the identification tool to determine the appropriate attribute value for the tower sites, open the attribute table, select this value, right click on the file, and use Save selection as to save a shapefile towersites.shp in the Lab3 folder. If you have done this correctly it should look like the map shown above. Again, you should now review your work, thinking about what the various geoprocessing tools have accomplished for you. Answers

6 Problem 6. The SQL command is a_a_value = 1 Explanation: The layer elfirevegegr contains polygons that are greater than 4000m from the fire station and on open land, but it contains areas below 300m as well as above 300m The SQL command selects these values above 300m (why?). Another way to do this analysis would be to use v.extract.where at the end of Problem 5 to extract polygons above 300 m.