GeoMedia Grid: How Do I: Find the Highest Elevation within an Area Topics: Key Words: Tower Location, Site Location, Raster to Vector, and Vector to Raster Digital Elevation Models, Buffer Zone, and Zonal Score.
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Table of Contents Finding the Highest Elevation Values within an Area...1 Data Requirements... 1 Creating the Buffer Zone... 1 Converting a Query into a GeoMedia Grid Layer... 2 Determining the Highest Elevation Value within Each Buffer Area... 3 Determining the Location of the Highest Elevation within Each Buffer Area... 5 Converting a Highest Elevation Location to a Feature Class... 7 Copyright 2006 Intergraph Corporation i
Copyright 2006 Intergraph Corporation ii
Finding the Highest Elevation Values within an Area Data Requirements Find the Highest Elevation within an Area In order to carry out this workflow, you need, at a minimum, a DEM in GeoMedia Grid format and either a set of points or areas in GeoMedia format. As can be seen from the following graphic, the GeoWorkspace in this case has a DEM and Point Feature Class containing two points. This How Do I illustrates using the Point Feature Class to create a 1000-meter buffer zone around each point. These buffer zones are then used as the polygons to determine the location of the maximum elevation. In this instance, this workflow was used to help locate the optimal site for locating a radio tower. The users in this case picked areas that they thought would be good locations to place a tower using GeoMedia. They then wanted GeoMedia Grid to locate the high points within the selected buffer zones. Creating the Buffer Zone 1. Choose Analysis > Buffer Zone to display the Buffer Zone dialog box. 2. Use the drop-down list to specify the Feature Class to which you want the buffer zone applied. In this case, the View_Point Feature Class is specified. 3. Specify a distance for the buffer zone distance. In this case, 1000 M is specified. 4. Provide a meaningful name to your Query. In this case Buffer Zone is specified. Copyright 2006 Intergraph Corporation 1
5. Should you want to see the results of the query graphically, check Display buffer zone in map window. The graphic below shows which parameters were used to illustrate this example. 6. Click OK to execute the Query. Once the command completes, the buffer zone query should be displayed in the GeoMedia Map window. This query will be used to determine the maximum elevation within 1000 meters of the two points within the Study Area. Converting a Query into a GeoMedia Grid Layer The Rasterize Legend Entry(s) command creates a new grid layer from either a vector Feature Class or from a query. The grid layer will have the same coordinate system, cell resolution, and extents as the current Study Area. 1. Select the Feature Class or Query from within the GeoMedia Legend window that you want to convert to a GeoMedia Grid layer. In this case, the Query called Buffer Zones is used. 2. Choose Grid > Layer > Rasterize Legend Entry(s). Copyright 2006 Intergraph Corporation 2
3. In this case, the legend entry is a query and, as a result, the Attribute Selection dialog box will be displayed. Select an attribute that makes sense for your application. In this case, the attribute used is Attribute1. Note: If you want the work-flow to report the highest elevation for each point in the Study Area, it is important to select an attribute that uniquely identifies each point. Click OK. A new grid layer with the same name as the query is created and added to the active Study Area. In this case, the GeoMedia Grid layer contains three zones, VOID, 2, and 6, where 2 and 6 are the attribute values extracted from Attribute1 during the Rasterization process. Your values will likely differ from those above. However, as noted earlier in this How Do I, it is important that each buffer area have a unique zone value. If the two areas have the same value, the work-flow described in this document will treat the two areas as a single area (i.e., two islands with the same value) and will therefore treat these separate areas as if they were touching one another. Determining the Highest Elevation Value within Each Buffer Area The Zonal Score command creates a grid layer of summary statistics for geographic zones. In this case, the DEM will act as the Data layer upon which the Maximum statistic will be applied, and the Buffer zone map will be used as the geographic zones map. When the Maximum statistic is specified in the Zonal Score command, the resulting layer data values are equal to the maximum of the Data layer data values in each zone defined by the geographic zones in the Source layer. Copyright 2006 Intergraph Corporation 3
1. Choose Grid > Statistical Analysis > Zonal Score. 2. For the Source layer, use the drop-down list to specify your buffer zone in the Layer name field. In this case, the grid layer named Buffer Zones is used. 3. For the Data layer, use the drop-down list to specify your DEM in the Layer name field. In this case, the grid layer named DEM is used. 4. Use the Statistic drop-down list to select the Maximum statistic. Note: If you were looking for the minimum value, you would make use of the Minimum statistic. 5. Enter a name that makes sense for the Result layer. In this case, the name Maximum Value Within Buffers is used. 6. Clear the Place results in map window check box. 7. Click OK to execute the command. A new grid layer with the Result layer name is created and added to the active Study Area. In this case, the GeoMedia Grid layer contains three zones, VOID, 1900.10, and 2354.00. Copyright 2006 Intergraph Corporation 4
The zone containing 1900.10 indicates the highest value found in zone 2 (i.e., the first buffer area), and the zone containing 2354.00 indicates the highest value found in zone 6 (i.e., the second buffer area). We now know the highest value that falls in the two buffer areas. We now need to determine the location of these values within the buffer area. Determining the Location of the Highest Elevation within Each Buffer Area The Calculator creates a new grid layer by applying mathematical and/or relational operations to one or more input grid layers. The operations are applied to the values of the cells of the input grid layer(s) in a process known as "layer math" or "overlay math." In this case, the Calculator command is going to be used to perform a Boolean operation that determines which DEM cells match the values in the Maximum Value Within Buffers map layer. 1. Choose Grid >Calculator. 2. When the Calculator opens, click on the Scientific radio button to open the full calculator window. 3. Use the facilities in the interface to create a statement that performs a Boolean test between your DEM layer and your Maximum Value Within Buffers map layer. In this case, the statement is DEM == Maximum Value Within Buffers. 4. Provide a meaningful name for the Result Layer name. In this case, the name Boolean Location of Maximum Value Within Buffers is used. 5. Click OK. 6. Use Grid > Layer > View Legend(s) to examine the results. Copyright 2006 Intergraph Corporation 5
The entire map should consist of two zones (i.e., 0 and 1). The zone with a value of 0 indicates a false condition (i.e., the value in DEM did not equal the value in the Maximum Value Within Buffer map). The zone with a value of 1 indicates a true condition (i.e., the value in DEM did equal the value in the Maximum Value Within Buffer map). It is possible that there could be more then one match for each of the zones; for example, if the DEM has two highest peaks in it that have the same elevation. In this example, this condition did not exist and, as a result, there are only two cells that met the Boolean criteria (i.e., one highest cell location for each buffer area). We now have the location of the highest elevation for each of the buffer zones. However, to fulfill the requirement of this workflow, the raster results need to be converted into a feature for display in the GeoMedia Map window. To accomplish this final task, the cells that did not meet the criteria need to be assigned a value of VOID. This can be done using the Grid > Classification > Recode command; however, since the Legend for the result map is open, it will be used to accomplish this task. 1. Click on the first legend entry (i.e., the legend that has a value of 0). 2. Right click to display the context menu. 3. Select Change Value(s) from the context menu. 4. When the Change Value(s) dialog box is displayed, change the value from 0 to VOID. 5. Click OK. Copyright 2006 Intergraph Corporation 6
The value for the first zone should change accordingly (i.e., the legend will contain two zones, VOID and 1). The map is now ready to convert into a vector Feature Class. Converting a Highest Elevation Location to a Feature Class This Vectorize to Feature Class command converts grid (raster) information to a vector Feature Class. The features are stored in the same connection as the current Study Area. Vectorized features can be edited and viewed in GeoMedia, exported to other applications, used to update data in the warehouse, or used to build queries. 1. Choose Grid > Layer > Vectorize to Feature Class. 2. Select your Boolean result map. In this case, Boolean Location of Maximum Value Within Buffers is used. 3. Set the Conversion type to Point. 4. Set the Use value as key option to No. If you were to select this option, that value in the legend would be used to create the record. For example, if you had a grid layer with four points (one with a value 5, one with a value 19, and two with a value of 20), the resulting Feature Class would create three records, not four. 5. Input a meaningful result feature class name for the new Feature Class. In this case, the name MaximumElevationPoints is used. Please note that Feature Class names cannot contain spaces. Copyright 2006 Intergraph Corporation 7
6. Click OK to execute the command. The points are automatically converted to a point-based Feature Class. The last step in this process is to display the point in the GeoMedia Map window. 7. Choose Legend > Add Legend Entries. 8. Select the check box of the newly created Feature Class. In this case, MaximumElevationPoints is selected. 9. Click OK to execute the command. 10. The points with the highest elevation for each buffer area should be displayed in the GeoMedia Map Window. Copyright 2006 Intergraph Corporation 8