The following workflow uses the Black Mountain.gmw file that was saved at the end of the Accessing Online Data topic.
Cropping a raster layer limits the geographic extent of the layer by removing the pixels outside a defined area. This extent can be determined using one of several methods, including entering the coordinate bounds, matching the extent of another layer, or, most commonly, by selecting an existing polygon.
The collection of images that we have been using in this course cover a rectangular area that is much larger than the extent of the town. In order to reduce the size of the imagery, we will create a bounding polygon derived from the geographic extent of the parcels, we will buffer this polygon to enlarge it slightly, and finally we will use the buffered polygon to crop the imagery.
- If necessary, open the workspace called Black Mountain Imagery.gmw, which was saved at the end of the last exercise.
- In the Control Center, enable the display of the Black Mountain Imagery tiles and the Black Mountain Parcels.shp and make sure the other layers are unchecked.
- Double-click on Black Mountain Imagery and click the Default button below the Color Intensity slider.
- In the Control Center, select (highlight) the Black Mountain Parcels.shp layer, right-click, and choose Layer > BBOX/COVERAGES.
- After confirming the selection of the parcels layer, choose the option to create a Polygonal Coverage Area.
The alternative Rectangular Area is often referred to as a Minimum Bounding Rectangle (MBR).
- In the Concave Hull Options dialog box, move the smoothing slider to Rough (Smoothing Factor: 1).
This ensures that the polygon will closely adhere to the outer boundary of the parcels.
- Click the OK button and note that a new vector layer has been added to the Control Center.
- Turn off both the imagery and the parcels to see the resulting coverage polygon.
Next, we will create a 50-meter buffer to enlarge this polygon.
- Enable the Digitizer tool (Alt+D) and select the coverage polygon.
- From the Digitizer (Edit) toolbar, click the Buffer button.
- In the Buffer Area Creation Setup dialog box, apply the following parameters:
- Feature Layer: Type ‘Buffer’
- Number of Buffer Zones: 1
- Buffer Distance: 50 meters
- Leave the other settings in their default state and click the OK button.
- In the Control Center, uncheck Black Mountian Parcels.shp (Coverage Polygon) and note the extended coverage area on the map. This buffer polygon will be used to crop the imagery.
- Using the Digitizer, select the buffer polygon on the map.
- In the Control Center, check the box next to Black Mountain Imagery and double-click on this layer to open the Raster Options dialog box.
- Select the Cropping tab, choose the option to Crop the Currently Selected Polygon(s), and click the OK button.
- Uncheck the Buffer layer in the Control Center and note the image layer is limited to the extent of the town.
- Save a new version of the workspace in your My Maps folder (File > Save Workspace As…) called Black Mountain Cropped Imagery.
This cropping process can also be applied to online data, which reduces the extent of the pixels that will be streamed.
- The simplest way to crop a raster layer is to create a polygon defining the area of interest
- A buffer can be used to enlarge the extent of the area to be cropped
- By reducing the number of pixels being displayed, cropping will increase the speed of any raster rendering or analysis processes
- Click here for more information on the image cropping methods available in Global Mapper.
This concludes the Working with Raster Data lesson. Why not take this quiz to see how much you learned?.