Tuesday, 31 March 2015
How to convert an ArcGIS flow accumulation layer into a "Boolean" stream layer
A flow accumulation layer is a continuous raster that stores the number
of upstream raster cells
that flow into each cell. As water accumulates in cells as it flows
downhill, higher accumulation values should correspond to larger
streams. To create vector stream channel lines from a flow accumulation
layer it must first be reclassified to a Boolean layer that defines if a
channel exists within each cell or not. To do this a flow accumulation
threshold must be set that defines how much flow a cell must have to be
identified as a 'channel'.
Boolean layers contain store one of two values in each cell that represent either 'true' or 'false' ('yes'/'no') in respect of the entity being modelled. In Boolean mathematics, "true" is assigned the value '1', and false '0'. But in some situations, including reclassifying an ArcGIS flow accumulation layer, data needs to classed '1' for 'true' and 'NoData' for 'false'.
In the HydroGIS practical, you create a flow accumulation layer from your hydrologically correct DEM. Flow accumulation ranges from 0 (no flow from no other cells flow into this cell) to 10119 (the number of upstream cells that flow into a cell).
To convert the flow accumulation surface into vector stream channel lines you must select a threshold flow accumulation value above which a cell is defined as containing a stream channel (channel = 'true'), and below which no channel is present (channel = 'false'). This threshold determines the density of stream lines that will be created. A higher threshold will result in fewer stream cells than a lower threshold.
A good way to select a threshold is to change the symbology of the flow accumulation layer so that it only displays cells with values above the threshold. You can then view the density of channel cells before creating your Boolean channel raster layer.
Open the flow accumulation Layer Properties and select the Symbology tab. Now set the layer to show two classified values. The class containing lower values represents 'false' cells that do not contain stream channels. The class containing higher values 'true' cells that contain channels. Select the Classify... button to change the threshold between the two classes.
See how a flow accumulation threshold to 3036 results in very few stream channel cells (above), whereas a threshold of 100 defines many more stream channel cells (below).
When you have chosen your threshold, use the Spatial Analyst Reclassify tool to create a 'Boolean' raster that stores '1' in channel cells and 'NoData' in non-channel cells. To do this you must change the 'New values' from the default incremental integer classes to '1' and 'NoData'.
The 'Stream to Feature' tool in the Hydrology tool set can now be used to generate vector streamlines from this 'channel layer' (Input stream raster) and the flow direction raster for the hydrologically correct DEM the stream lines were derived from.
Boolean layers contain store one of two values in each cell that represent either 'true' or 'false' ('yes'/'no') in respect of the entity being modelled. In Boolean mathematics, "true" is assigned the value '1', and false '0'. But in some situations, including reclassifying an ArcGIS flow accumulation layer, data needs to classed '1' for 'true' and 'NoData' for 'false'.
In the HydroGIS practical, you create a flow accumulation layer from your hydrologically correct DEM. Flow accumulation ranges from 0 (no flow from no other cells flow into this cell) to 10119 (the number of upstream cells that flow into a cell).
To convert the flow accumulation surface into vector stream channel lines you must select a threshold flow accumulation value above which a cell is defined as containing a stream channel (channel = 'true'), and below which no channel is present (channel = 'false'). This threshold determines the density of stream lines that will be created. A higher threshold will result in fewer stream cells than a lower threshold.
A good way to select a threshold is to change the symbology of the flow accumulation layer so that it only displays cells with values above the threshold. You can then view the density of channel cells before creating your Boolean channel raster layer.
Open the flow accumulation Layer Properties and select the Symbology tab. Now set the layer to show two classified values. The class containing lower values represents 'false' cells that do not contain stream channels. The class containing higher values 'true' cells that contain channels. Select the Classify... button to change the threshold between the two classes.
When you have chosen your threshold, use the Spatial Analyst Reclassify tool to create a 'Boolean' raster that stores '1' in channel cells and 'NoData' in non-channel cells. To do this you must change the 'New values' from the default incremental integer classes to '1' and 'NoData'.
The 'Stream to Feature' tool in the Hydrology tool set can now be used to generate vector streamlines from this 'channel layer' (Input stream raster) and the flow direction raster for the hydrologically correct DEM the stream lines were derived from.
Friday, 6 March 2015
How to find flat triangles in a TIN
Flat triangles and edges are common artifacts in triangular irregular network (TIN) surfaces created from contour line. They occur where the three nodes of a triangle, or the two nodes of a triangle edge, are derived from the same elevation contour. They are artifacts of the model which do not represent the true elevation at these places.
Flat triangles and edges can be located in a number of ways in ArcGIS.
Flat triangles and edges can be located in a number of ways in ArcGIS.
- Display the original contour lines or the TIN soft-edges over the TIN model and look for triangles and edges in with nodes derived from contour lines with the same elevation.
- Use the Information tool to explore how values change within a triangle or along an edge. If all values are the same it is flat.
- Display the TIN in ArcScene and look for "flat looking" triangles. Confirm with the information tool.
- Render the TIN to show slope. In the TIN Symbology tab Add... a Face slope with graduated color ramp renderer. Now change to symbology levels to differentiate triangles with very low slope from those with a slope.
The red triangles are flat or very nearly flat.
ArcGIS Background Processing error
If a tool returns a Background Processing error then you should try running the tool with background processing turned off.
To turn off background processing select Geoprocessing Options... from the Geoprocessing menu and, in the dialog, uncheck the Enable Background Processing checkbox.
Disabling background processing means that you cannot work in ArcGIS until tools have finished working. A minor issue compared to the tool not working at all. If this does not fix the problem then report it to the KU IT Helpdesk and the module leader.
To turn off background processing select Geoprocessing Options... from the Geoprocessing menu and, in the dialog, uncheck the Enable Background Processing checkbox.
Disabling background processing means that you cannot work in ArcGIS until tools have finished working. A minor issue compared to the tool not working at all. If this does not fix the problem then report it to the KU IT Helpdesk and the module leader.
ArcGIS: Tool Not Licensed error
You will get a "Tool Not Licensed" error when trying to use an ArcGIS Tool, such as Create TIN or Flow Direction, and the ArcGIS Extension that the tool is part of is not enabled.
To use the tool you must active the extension. The error dialog should inform you which extension the tool is part of. In this case the Flow Direction tool is part of the Spatial Analyst extension, whereas the Create TIN tool is part of the 3D Analyst extension.
To enable extensions, select Extensions.. from the Customize menu and check the extensions that you need. The tool should now work.
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