Understanding height differences is crucial in various geographical analyses. Whether you’re in urban planning, environmental monitoring, or disaster management, the ability to Compare Height effectively provides valuable insights. This article delves into a powerful method for comparing heights relative to specific source points using Geographic Information System (GIS) tools, specifically focusing on ArcGIS. We will explore how to calculate and visualize height offsets, offering a comprehensive guide for GIS professionals and enthusiasts.
Understanding Relative Height Comparison in GIS
In GIS, comparing heights often involves analyzing elevation data to understand terrain variations. A common task is to determine how much higher or lower a location is relative to a set of reference points. This is particularly useful when you need to analyze areas around specific features, such as monitoring the height difference around buildings, trees, or geological formations. This process goes beyond simply looking at absolute elevation values; it provides a relative height comparison that highlights local variations and contextualizes elevation data.
Tools for Comparing Heights in ArcGIS
ArcGIS offers robust tools to perform this type of height comparison. Two primary tools stand out for creating buffer regions based on height and calculating relative height offsets: Euclidean Allocation and Distance Allocation.
Euclidean Allocation for Height Comparison
The Euclidean Allocation tool is effective for creating buffer zones around source points and allocating areas based on proximity. To use it for comparing heights, you can leverage its ability to consider source attributes.
- Height-Based Buffering: Assign an integer field to your source features representing their rounded heights.
- Maximum Distance Parameter: Utilize the Maximum Distance parameter to define the buffer radius.
- Source Field Specification: Crucially, specify the height field as the
source_field. This instructs the tool to consider the height attribute during allocation.
By configuring these parameters, the Euclidean Allocation tool generates buffer regions where each cell is allocated to the nearest source, taking into account the height of that source.
Distance Allocation for Surface Height Comparison
For scenarios where surface distance is important, especially in hilly or mountainous terrain, the Distance Allocation tool provides a more accurate approach to compare height. This tool calculates true distance over a surface, offering a significant advantage over planar distance measurements.
- Surface Distance Buffers: The Distance Allocation tool uses the Maximum Accumulation parameter to set the buffer radius, similar to the Maximum Distance in Euclidean Allocation.
- Height as Source Attribute: Again, specify the height field of your source features as the
source_field.
The key benefit of Distance Allocation is its ability to calculate buffers based on true surface distance. Imagine measuring a buffer uphill versus measuring it on a flat plane; surface distance accounts for the terrain’s undulation, providing a more realistic buffer zone, as illustrated below.
Visual representation of cell allocation to sources based on height, showcasing how different shades represent elevation relative to source points.
Visualizing Height Differences Relative to Sources
Once you’ve used either Euclidean or Distance Allocation to create height-aware buffer regions, the next step is to visualize the height differences. This involves subtracting the allocation raster from your original elevation surface.
- Subtraction for Offset Calculation: Subtract the output raster from the allocation tool (representing height-allocated zones) from your elevation surface raster. This operation yields a raster where each cell value represents the height difference relative to its allocated source.
- NODATA Areas: Areas outside the buffers will be represented as NODATA, effectively isolating the areas of interest around your sources.
- Visualization Techniques:
- Reclassification: Reclassify the resulting raster to categorize height differences into meaningful classes (e.g., significantly higher, slightly higher, same elevation, slightly lower, significantly lower).
- Stretch Renderer: Employ a multi-part stretch renderer, setting the midpoint interval to zero. This allows you to use a diverging color scheme where:
- Colors like red represent areas below the source elevation.
- Colors like green indicate areas above the source elevation.
- White or neutral colors show areas at approximately the same elevation as the sources.
This visualization clearly displays areas that are relatively higher or lower than the source points, enabling effective comparison of heights across the landscape.
Comparison of buffer regions calculated using planar distance (blue circle) and true surface distance (red-white-green buffer), highlighting the difference in buffer extent on sloped terrain.
Advanced Considerations for Height Comparison
Beyond the basic workflow, consider these advanced points for more nuanced height comparisons:
- True Surface Distance: As mentioned, the Distance Allocation tool’s capability to use true surface distance is invaluable for accurate buffer creation and subsequent height analysis, especially in varied terrain. This ensures that distance measurements are realistic and reflect field conditions.
- Mean Value Comparison: For comparing heights relative to the average height within each buffer zone, the Zonal Statistics As Table tool with overlapping zones capability can be used. This approach involves generating vector buffers, calculating the mean elevation within each buffer, and then proceeding with the subtraction method. However, note that this method might not leverage surface distance buffers.
- Data Type Considerations: While the allocation tools work best with integer source height attributes, you can use object IDs as allocation values and then join and lookup techniques to re-introduce floating-point height values if needed for higher precision.
Conclusion
Comparing heights in GIS using tools like Euclidean Allocation and Distance Allocation offers a powerful method for relative elevation analysis. By understanding and applying these techniques in ArcGIS, you can effectively analyze height differences, visualize terrain variations relative to source features, and gain deeper insights from your elevation data. Whether you need to assess the impact of infrastructure, analyze environmental changes, or plan urban developments, the ability to accurately compare height is a fundamental aspect of spatial analysis.
