6. Pre-processing: Steps taken to cut the DEM - To Study a small River Basin
From GeoMod
The DEM which we generally get or which we download are big and voluminous. Often our study area is small and to make things manageable and faster, it becomes necessary to clip the Raster Image.
I downloaded the Raster Image of Pearl River County but it was too big. I wanted to concentrate of one of the tributaries of Pearl River. Hence I followed the following steps to delineate a smaller area from the BIG raster data.
First of all we will create a polygon which would define the boundary of the data on which we want to work on.
Creation of Personal Geodatabase
1. Open ArcCatalog
2. Right click on the folder where you want to create your Personal Geodatabase > New > Personal Geodatabase.
3. Enter a name for your Geodatabase and open it.
4. Right click in the empty space > New Feature Class.
5. A new window will open which will ask for the name of new feature class. Enter the name and click next.
6. Leave the default database storage configuration and click next.
7. Click ‘Geometry’ in the new window which opens.
8. Click the ellipses (i.e. the "...") to the right of Spatial Reference | Unknown in the bottom table (Field Properties)
9. Click the Select button and choose the coordinate system/projection/datum.
10. Now click the X/Y Domain tab and set the MinX, MaxX, MinY and MaxY coordinates for the geographic extent of the new Feature Class.
11. Click Finish to save these settings for the template Feature Class.
Clipping the Raster Image according to need / purpose
1. Add the DEM and the polygon (from the Personal Geodatabase) to the data view.
Original DEM of Pearl River County, Mississippi
2. Activate the Editor bar by clicking View > Toolbars > Editor
3. Click Editor > Start Editing to start editing. A new window will open which will allow you to select the feature you want to edit.
4. Click the pen, just next to editor and start making the polygon as per your need.
5. Go to Editor > Save Edit > Stop Editing
DEM of Pearl River with the new Polygon
6. Now you are ready to clip the BIG raster image. Go to Arctoolbox > Conversion Tools > To Raster > Feature to Raster.
7. Enter the feature name, field, output raster and output cell size. Click OK
8. A new raster file would be displayed in the view which would be of the same shape as the polygon. All the cells in this rasterised grid have a value of 0.
9. Now we will do some raster calculations. For this first of all we need to change the values of the new raster to 1. For this go to ArcTools > Spatial Analyst Tools > Math > Plus
10. Enter the new raster’s path and name in ‘Input Raster.’ Enter 1 in the seond column and give a name for the output raster. Click OK. A new raster image would be created whose value would be 1.
11. Now we would multiply the original and big raster data of Pearl River with the new raster data with value 1. Click ArcTools > Spatial Analyst Tools > Math > Times. Enter the names of the names of both files and click OK.
Clipped Pearl River
12. And here comes your clipped image with all the features of the original DEM… the only difference being that it represents a smaller area and the area of your interest.
Try this out and let me know if you face any problems.
