Lab 9 Raster Analyses Tomislav Sapic GIS Technologist Faculty of Natural Resources Management Lakehead University
How to Interpolate Surface Turn on the Spatial Analyst extension: Tools > Extensions > check-off Spatial Analyst. 1. In ArcMap, make sure that Spatial Analyst is turned on in Customize>Extensions. If desired, set the Current and Scratch Workspace in Geoprocessing > Environment Settings > Workspace. 2. Open ArcToolbox and go to the Spatial Analyst Tools > Interpolation and select one of the methods (Inverse Distance Weighted, for example). A new raster, containing interpolated elevation values for the whole point DTM area, is created. 3. In the IDW window, set the layer that contains the DTM (DEM) points, the Z value field, the output cell size, and the location and name of the output raster. If desired, change other parameters, or otherwise, leave them as default. Click OK.
Surface Raster The product of the point DTM interpolation is a DTM raster, representing the surface interpolation for the whole point DTM extent. Elevation values for the area under and around the DTM points are interpolated and stored in a raster dataset. Raster elevation values can be classified into defined intervals of, for example, 10: go to layer properties>symbology tab>show: Classified>Classify button>classification Method: Defined Interval>Interval Size: 10. Information on the raster resolution (cell size), data depth, number of bands, and so on, can be found in Layer Properties>Source tab.
Raster Display Resampling Rasters can be displayed so that boundaries between different legend intervals (edge cell boundaries) are clearly distinguished or so that there is a continuous, imperceptible transition between them. The first method is used with rasters containing discrete values (e.g., values representing classes) and the second with rasters containing continuous values (e.g., elevation values).
Hydrology Modelling How to Create Stream Orders in ArcMap and Display Results 1. Run the Fill function in Spatial Analyst Tools>Hydrology on the DEM raster. The Fill function fills out sinks. Sinks are micro depressions where a cell is surrounded with cells of equal or higher values. The output becomes the final DEM raster. 2. Run the Flow Direction function (tool) by using the Fill output raster as the Input Surface Raster. You can leave other optional parameters as default. 3. Use the output raster from the Flow Direction function to run the Flow Accumulation function.
How to Create Stream Orders (cont d) The Flow Accumulation output raster contains values indicating how many cells are flowing into the each cell in the raster. In order to identify the streams, cells with higher values (e.g. > 100) can be selected as highly probable streams. All other cells can be assigned a NoData value. This can be done by using the Con function in the Conditional toolbox. Flow Accumulation raster 4. Use the Con function to assign values of 1 to all cells with Value > 100 and NoData (empty input box) to all other cells. This creates a stream raster. For each cell in the input (Flow Accumulation) raster: if value > 100 then assign a value of 1 in the output raster, otherwise, assign a value of NoData in the output raster. This creates a stream raster.
Stream raster How to Create Stream Orders (cont d) 5. In the Stream Order function, the stream raster is then used along with the Flow Direction raster to create a stream order raster. Use the Strahler method. Streams can be of 1 st, 2 nd, 3 rd, 4 th, order, and so on. 1st order streams flow into 2 nd order streams, which in turn flow into 3 rd order streams (if availabe), and so on.
How to Create Stream Orders (cont d) Once created, the stream orders raster can be overlayed with a shapefile containing real streams and the two can be visually compared. Real streams can be shown with a dotted line to enable seeing the stream order raster underneath.
How to Create a Viewshed Model A viewshed model represents the area that is visible from the specified point(s) or line(s). The points and lines can be entered into the modelling as raster or vector data models. If points are used, they can be either created anew or can be taken from another point dataset, such as a point DTM. In order to take a number of points from a point DTM, chosen points can be selected with the select tool, and a new shapefile created (right-click on the point DTM layer>data>export Data>Export: Selected Features; set the name and location for the new shapefile. 1. Create a point file containing the points that will be used as observer features. 2. Invoke the Viewshed tool from ArcToolbox>Spatial Analyst Tools>Surface. 3. Set your surface raster as the input raster, the observer point shapefile as the input observer features; specify the output raster. Leave the optional parameters as default and press OK.
How to Create and Display a Viewshed Model The raster output from the Viewshed function contains values of 0 and > 0. Zero represents not-visible, and > 0 visible (the actual number represents the number of observer points from which that particular cell can be seen). If created by invoking the tool from within ArcMap, the viewshed raster should have a visible/not visible legend applied to it. If ArcMap doesn t display the viewshed raster in two classes, Not Visible and Visible (0 and > 0), this can be done manually through the layer symbology. 1. Go to the Unique Values symobology setting. 2. Group > 0 values into one item; that leaves the 0 values as a separate item. 4. Label Not Visible and Visible items. Press OK. 3. Set the colours for the two categories: No Colour for not visible and a prominent colour for visible cells.