CHAPTER 2 GEOREFERENCING AND SHAPEFILE CREATION

Transcription

1 CHAPTER 2 GEOREFERENCING AND SHAPEFILE CREATION Georeferencing is the process of assigning real-world coordinates to each pixel of the raster. These coordinates are obtained by doing field surveys - collecting coordinates with a GPS device for few easily identifiable features in the image or map. Georeferenced data helps to view, query, and analyze with other geographic data. 2.1 GEOREFERENCING TOPOSHEET Exercise: 2.1 Georeferencing a scanned toposheet using QGIS. 1. Open QGIS and select Gereferencer tool from Raster option in the main menu. 2. You will get a georeferencer window, in that add the toposheet from the file location C:\Training\DAY 2\Data. 3. Assign coordinate points on the map. If you look closely, you will see coordinate grid with markings. Using this grid, you can determine the X and Y coordinates of the points where the grids intersect. Click on Add Point in the toolbar. In the pop-up 1

2 window, enter the coordinates. Remember that X=longitude and Y=latitude. Click OK 4. Similarly, add at least 4 GCPs covering the entire image. The more points you have, the more accurate your image is registered to the target coordinates. 5. After giving control points click on Settings icon select Transformation settings. 6. In the Transformation settings dialog box, choose the Transformation type Polynomial 1, Resampling method Nearest neighbor 7. Name your output raster as Georeferenced.tif. 2

3 8. Search for EPSG: 4326 in the filter box in the Target SRS option. 9. Make sure that the Load in QGIS when done option is checked. 10. Save to the file location C:\Training\DAY 2\Output\Georeferenced.tif 11. Click Start georeferencing from the File option in the menu of Georeferencer window. o This will start the process of warping the image using the GCPs and creating the target raster. 12. Once the process finishes, georeferenced layer loaded in QGIS Display Window. Georeferencing is now complete. But as always, it s a good practice to always verify your work. 13. Right click on the layer. Select properties. Go to Transparency option and set it in to 60%. 14. Click OK. You will see the Georeferenced Toposheet overlaid with Google Hybrid layer. 3

4 2.2 GEOREFERENCING CADASTRY Exercise: 2.2 Georeferencing a Scanned Cadastry using openlayer plugin. 1. You are given the scanned cadastry of Kalliyoor village in C:\Training\DAY 2\Data folder. 2. Open QGIS and add the Google Hybrid layer to the Map canvas (Plugin > Open layers plugin > Add Google Hybrid ). 3. Locate the area of interest using Geosearch plugin (it can be installed from Plugins 4

5 Manage and install Plugins option) by typing Kalliyoor Village. 4. Select Gereferencer tool from Raster option in the main menu. 5. You will get a georeferencer window, add the Scanned Cadastry from the specified folder. 6. Identify the places in Google Hybrid map which corresponding to the Cadastry. 7. Select GCP from the cadastry which is identified on the base layer. 8. Assign coordinate points on the map. Since we do not have any reference point, we collect from the Google map. 9. Click on Add Point in the toolbar. In the pop-up window, Click on from map canvas. 5

6 10. You will direct to Map pane where the Google Hybrid layer is already opened, Search for the respective place location in reference map. 11. Location of the corresponding area will be fetched from the base layer. 12. Similarly, add GCPs covering the entire cadastry. The more points you have, the more accurate your image is registered to the target coordinates. 13. After giving control points click on Settings icon, select Transformation settings. 14. In the Transformation settings dialog box, choose the a. Transformation type Polynomial 2 b. Resampling method Nearest neighbor 15. Name your output raster as Cadastry_Georeferenced.tif 16. Choose EPSG: Make sure the Load in QGIS when done option is checked. 17. Save to the file location C:\Training\DAY 2\Data\Output 18. Click Start georeferencing from the File option in the menu of Georeferencer window. a. This will start the process of warping the image using the GCPs and creating the target raster 19. Once the process finishes, Georeferenced layer loaded in QGIS Map pane. 20. Right click on the layer. Select properties. Go to Transparency option and set it in to 6

7 60%. 21. Click OK. You will see the Georeferenced cadastry overlaid with Google Hybrid layer. DIGITIZING LAYERS Spatial data and its attribute data are stored in the computer in different ways depending on the GIS software in which it was created. Geographical data can be recorded directly from the field in a digital format by using devices and Global Positioning Systems (GPS) and satellite imagery. Data also can be input from analogue format (hard copy) by mean of digitizing or scanning. Spatial (points, lines or polygons) and attribute data are added to shapefile by using various methods. Data entry by digitizing: We can create points, lines and polygons by tracing the features from an image, which is placed in QGIS as a background layer.after spatial reference is defined, the data can be edited using editor tool bar in QGIS. 7

8 2.3 SHAPEFILE CREATION Exercise: 2.3 Create Shapefile using Cadastral map in QGIS 1. Open QGIS and add georeferenced cadastral map from file location C:\Training\DAY 2\Output\ Cadastral_Referenced.tif 2. In order to start digitizing, first create a new shapefile using New Shapefile Layer option in Layer in the main menu tab Capture Point Features: 3. A New vector layer window pops up. Select the vector layer type as point and specify CRS as EPSG: Add attribute fields using Name option in the New vector layer window. 5. Since it s a point layer, we will have attributes such as Asset_Name and Asset_Type. 6. Select Type as TEXT and give appropriate width for the word length. 7. Click Attribute List box and select Add to attribute list and click OK. 8. When you click OK, it will ask to save. Browse 8

9 the directory C:\Training\DAY 2\Data\Output to save the point shapefile. 9. Give the layer name as Assets in the output folder and save. 10. On saving the asset layer is automatically added to the QGIS. 11. Once the Asset layer is loaded, click the Toggle Editing button to put the layer in editing mode. 9

10 12. Select Add Feature tool on the toolbar and select a point using mouse button at a point as close to the centre of the point feature as possible. 13. Place the add feature icon on the assets selected and click. 14. On click, a window with title Feature Attributes is opened and enter values for different attribute fields. 10

11 15. Repeat the same operation for the capturing other point features. 16. After the completion of the task, go to Editor Toolbar and click Save Layer Edits to save all the captured features. 17. Click Toggle editing once again to exit the task. Points are used to represent features that are too small to be represented as areas at the scale of mapping being used, such as individual houses, postbox, a tree, etc. Digitize line features: 1. Create a new shapefile for line representing the roads using New Shapefile Layer option in Layer in the main menu tab. Note: Shapefile creation for point, line and polygon is same and only the difference is the selection of Type of vector. For eg, select Type point for locations, line for linear features and polygon for area features. 2. Select line as the Type. 3. Specify the CRS as EPSG: Create new attribute called Road_Name and Road_Type. 5. Save the layer as Road to C:\Training\DAY 2\Data\Output path. 6. Once the Road layer is loaded, click the Toggle Editing mode. button to put the layer in editing When working with lines it is often necessary to join them. For example roads join at intersections. When digitizing roads, it shares common points at their intersections. To ensure this happens you need to snap points to one another. 11

12 7. To enable snapping go to the settings in the main menu and select Snapping options. 8. On selecting snapping option, a snapping window will appear and check Road layer. 9. Choose to vertex as mode and 10 as tolerance. 10. Check Enable snapping on intersection and click apply. 11. Select Add feature button and click on the map canvas to add a line vertex. Digitize the line representing the road. 12. Once you have digitized a road segment, right-click to end the feature. 12

13 13. Enter attribute information to the Feature Attributes window. 14. Repeat the same operation for the capturing other point features. 15. After the completion of the task, go to Editor Toolbar and click Save Layer Edits to save all the captured features. 16. Click Toggle editing once again to exit the task. Digitize polygon features: 1. Create a new shapefile for Polygon layer representing blocks in the cadastral map. 2. Give attribute fields as Survey_No and Remarks. 3. Save Polygon shapefile as Cadastry in C:\Training\DAY_2\Data\Output. 4. To enable snapping go to the settings in the main menu and select Snapping options. 5. On selecting snapping option, a snapping window will appear and select Snapping Mode to Advanced. 6. Choose layer mode to vertex and segment. 7. Check Enable snapping on intersection and Avoid intersection, click OK. 8. Click Add feature button and click on the map canvas to add a polygon vertex. 9. Digitize the polygon representing the survey_no and right-click to finish the polygon. 10. Feature Attributes window will open where you can give attribute information. 13

14 11. Click Save Layer Edit option in the editor tool bar after completion of the task. Polygons are usually used to represent features such as areas, dams, and other features at a relatively larger scale when compared to point. 2.4 SHAPEFILE CREATION USING OPENLAYERS Open Layers plugin allows you to add a number of image services from Google, Bing, Yahoo and OpenStreetMap to the map canvas. Georeferencing method involved reading the coordinates from your scanned map and inputting it manually. Many times though you may not have the coordinates printed on your map, or base image for digitizing. In that case, you can use another georeferenced data open layer data source as your input for creating layer. 14

15 Exercise: 2.4 Shapefile creation using openlayers 1. Add the vector layer of Kerala district shapefile. 2. Go to plugin openlayer pluginadd Google satellite layer. 3. District shape file has been correctly overlaid on the Google satellite layer. 4. Create a new Vector layer River from Layer menu New shapefile layer. 5. Start digitizing the Vector layer 15