Measure the Perimeter of Polygons Over a Surface. Operations. What Do I Need?

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1 Measure the Perimeter of Polygons Over a Surface Operations What Do I Need? Incremental Frontage Score To measure the perimeter of polygons over a surface you need to have two input map layers. The first map layer should contain your areal features. The areal features can have any value. The areal features could be land uses such as soil types, crop types or forest types. The second map layer should be a DEM. This map layer provides the elevation information needed accurately calculate perimeter over a surface. The DEM map layer must have the same cell resolution and geographic coverage as the areal feature map layer. Example Measure the Perimeter of a Polygon Over a Surface The Incremental Frontage operation is used to accurately measure the perimeter length of each boundary cell that makes up an areal feature. The results of the Incremental Frontage operation can then be used as input to the Score operation to determine the total perimeter of each unique areal feature. Assign a unique value to each unique areal feature in the map layer before using Score. Using these two operations together allows you to accurately determine the perimeter of areal features that traverse hilly, mountainous, or other nonplanar regions. HD-FRT-1

2 To measure the perimeter of a polygon over a surface, start with a map layer that represents the areal features you want measured. In this example, a soils map of Huntsville, Alabama, USA is used. Each soil type has been assigned a unique value; there are 123 unique values, or zones, in this map layer. Next, you will need an elevation map. In this example, a United States Geological Survey (USGS) DEM (Digital Elevation Model) of Huntsville, Alabama will be used. This map was imported into MFworks using the SDTS (Spatial Data Transfer Standard) translator module. The elevation on this map layer range from approximately 170 metres to just over 500 metres. HD-FRT-2

3 Step One: Determine Incremental Frontage The MFworks Incremental Frontage operation calculates the incremental perimeter length of each boundary cell of an areal feature based upon the cell resolution, inferred shape, and surface elevation supplied by the input map layers. If you were to perform this operation from the Script window the statement would be: "Huntsville Inc_Frontage" = IncrementalFrontage "Hunstville Soil Type" Elevation "Huntsville DEM"; HD-FRT-3

4 The following map results from the above Incremental Frontage operation: This map layer contains a matrix of cells where each cell has been encoded with a value that represents its perimeter length. In this example, the input maps had a cell resolution of 30 metres and therefore the results are in metres. If you examine the 5 cell by 5 cell matrix of the numeric magnifying glass above, you will notice that the centre cell has been encoded with the value This value represents the perimeter length of that cell. Nonboundary cells are encoded with the value 0. The key to the Incremental Frontage operation is that perimeter length values are based upon the relative location of neighbouring cells in the input map layer and the elevation information supplied by the DEM map layer. This incremental information can be used to help determine the total perimeter of each soil type in the Huntsville area. Step Two: Determine Total Perimeter of Each Soil Type The Score operation is one of the most powerful area based spatial analysis operations in Raster GIS. It is used to create a map layer of summary statistics based on cell values found within a region or area. HD-FRT-4

5 In this example, the Score (Total) operation is used to report the total of all the values that are found within each soil class. If you were to perform this operation from the Script window the statement would be: "Huntsville Total Perimeter" = Score "Hunstville Soil Map" By "Huntsville Inc_Frontage" Total; The resulting map layer contains the original soil class polygons; however, each cell within the soil class has been encoded with the total perimeter value for the entire class. If you examine the 5 cell by 5 cell matrix of the numeric magnifying glass below, you will notice that the centre cell has HD-FRT-5

6 been encoded with the value This value represents the total area in metres for this soil polygon. The matrix depicts the area for two of the 123 soil classes. The areas values are and respectively. This type of information is useful when comparing crop yields based upon soils classes. HD-FRT-6