Comparing the height of terrain relative to specific source points is a common task in geographic analysis. This article will guide you through using ArcGIS tools to achieve this, focusing on height comparison using Euclidean Allocation and Distance Allocation tools. These methods allow you to generate buffer regions around source points based on height and then calculate height differences relative to these sources.
To effectively perform height comparison, we will utilize either the Euclidean Allocation or Distance Allocation tools in ArcGIS. These tools are instrumental in creating buffer regions around your designated source points.
Method 1: Utilizing Euclidean Allocation for Height Comparison
The Euclidean Allocation tool is effective for allocating cells to the nearest source based on Euclidean distance. To use it for height comparison, follow these steps:
- Prepare Source Features: Ensure your source point features have an integer field representing their heights. If your height values are not integers, round them to the nearest integer.
- Run Euclidean Allocation: Open the Euclidean Allocation tool in ArcGIS.
- Input your source features.
- Specify the elevation raster as the input raster for allocation.
- In the environment settings, ensure your processing extent and cell size are appropriately set to match your elevation raster.
- Crucially, use the Maximum Distance parameter to define the buffer radius around your source points. This radius determines how far from each source point the allocation will extend.
- Set the height field (the integer field you created) as the source_field. This tells the tool to consider the height values during allocation.
Method 2: Leveraging Distance Allocation for Height Comparison
The Distance Allocation tool offers similar functionality but with the added capability of calculating true distance over a surface, making it particularly useful for height comparison in varied terrains.
- Prepare Source Features: As with Euclidean Allocation, ensure your source point features include an integer field representing their rounded heights.
- Run Distance Allocation: Open the Distance Allocation tool in ArcGIS.
- Input your source features.
- Specify your elevation raster as both the input raster and the surface raster. Using the elevation raster as the surface raster enables the calculation of true surface distance.
- Use the Maximum Accumulation parameter to set the buffer radius, similar to the Maximum Distance in Euclidean Allocation.
- Designate the height field as the source_field.
Calculating and Visualizing Source-Relative Height Offsets
Once you have run either Euclidean Allocation or Distance Allocation, you will have a raster representing the allocated regions around each source point, considering their heights. To complete the height comparison and visualize the height offsets:
- Subtract Allocation Raster from Elevation Surface: Use the Raster Calculator tool to subtract the output raster from the Euclidean or Distance Allocation from your original elevation surface. This operation calculates the height difference between each cell and its allocated source point’s height.
- Visualize the Results: The subtraction output will show NODATA values in areas outside the buffer regions, which is the desired outcome. To visualize the height comparison effectively:
- Use a multi-part stretch renderer for the output raster.
- Set the middle interval point to zero.
- Choose a color scheme where:
- Shades of red represent cells with elevations below their source points.
- Shades of green represent cells with elevations above their source points.
- White represents cells at approximately the same elevation as their source points.
This visualization clearly depicts areas that are relatively higher or lower than the source points, achieving a comprehensive height comparison.
Image: Visual representation of height comparison using allocation and stretch renderer, showing cells colored relative to source point heights.
Additional Considerations for Advanced Height Comparison
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True Surface Distance with Distance Allocation: A significant advantage of the Distance Allocation tool is its ability to calculate true surface distance. This is crucial for accurate height comparison in hilly or mountainous terrain where horizontal planar distance underestimates the actual distance along the surface.
Image: Comparison of buffer regions calculated using planar distance (blue circle) versus true surface distance (red-white-green buffer), highlighting the difference in buffer extent on uneven terrain.
As illustrated, buffers calculated using true surface distance (red-white-green) more accurately reflect real-world distances compared to planar distance buffers (blue circle), especially on slopes. For precise height comparison, especially in non-flat areas, Distance Allocation with surface distance is highly recommended.
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Height Comparison Relative to Mean Buffer Height: For scenarios where you need to compare heights relative to the average height within each buffer region, you can utilize the Zonal Statistics As Table tool with overlapping zones.
- First, generate buffer zones around your source points using the Buffer tool (vector-based). Ensure these buffers can overlap.
- Use Zonal Statistics As Table to calculate the MEAN elevation within each buffer zone.
- Join this mean height value back to your source points.
- Proceed with the subtraction method described earlier, using the mean height instead of the original source point height.
While this method provides height comparison relative to the average buffer height, it does not incorporate surface distance in buffer calculations.
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Handling Floating Point Heights: If using integer heights is not suitable, and you need to maintain floating-point precision, you can use the object ID of your source points as the value to spread in Euclidean or Distance Allocation. Subsequently, use Join and Lookup tools to transfer the original floating-point height values back to the output raster for accurate height comparison.
Conclusion
ArcGIS provides powerful tools for conducting detailed height comparison relative to source points. By employing Euclidean Allocation or Distance Allocation, and understanding the nuances of surface distance and data handling, you can effectively analyze and visualize height differences in your spatial data. Choosing between Euclidean and Distance Allocation depends on the terrain and the required accuracy of your height comparison. Distance Allocation, especially with its surface distance capability, offers a more realistic approach for varied landscapes.
