Integrating Hillshade and Vegetation
February 24 2009 |
I am making a topographic map of Australia with their new DEM. I have created a simple hillshade grid and am experimenting with transparency. Which layers should be transparent? I started with the vegetation polygon layers and then overlaid the hillshade with 50% transparency. However this is too dark. So I have tweaked a 2 StdDev symbol table to be extremely stepped. 0 - 160 -> 0, 172 - 255 -> 255. This enables just the shadows to be grey and all other areas to be white.
But is there a way of altering the vegetation brightness instead of adding a transparent overlay? I vaguely remember combining grids in a stack to set the brightness.
I have tried the Swiss equations, but they crash on my datasets. Do I have to do it in small tiles?
Mapping Center Answer:
OK – so you asked a number of questions – here are the various answers:
RE: the order of the layers:
- At smaller scales it is probably a better idea to put the polygon layer (like vegetation) over the hillshade – the reason is that there will be so little variation in the hillshade at this scale that you won’t really need to worry about how the tonal variations in the grayscale color ramp used to symbolize the hillshade will affect the appearance of the overlaid polygonal layer.
- At larger scales, it’s different. For these kinds of maps, you will see more detail in the hillshade terrain. If you set the transparency of the hillshade to about 60% and leave the layers under them non-transparent, then the color of the polygon layer under the hillshade actually helps to accentuate the tonal variations in the terrain surface.
Now, if you are making maps at BOTH these scales and you want them to look very similar, then you can use the Eyedropper tool to determine what the RGB (red, green, blue) values are for the colors you have at the larger scale. Once you have these, you can use this as the color definition for the polygon layer that is placed over the hillshade terrain at smaller scales. You may have to adjust the color slightly – it is easiest to do this with the HSV color model so that all you need to vary is either value (lightness-darkness, and/or saturation (intensity of color). Note that to change the color model all you need to do is click the arrow at the upper right of the Color Selector box.
RE: the darkness of the hillshade and transparent overlay:
Check out this blog entry on how you can modify the color ramp used to display the hillshade so that your overlaid data shows up better: Hillshades for analysis maps. This will allow that low lying areas (with very little relief) to show pretty much ONLY the polygon colors. The areas with higher relief will have the gray tones that indicate terrain variation.
RE: the Swiss hillshade tools:
The Swiss hillshade tools should work even for larger extents, so I am not sure why you are having a problem. You might just try doing the following set fo raster procedures to create the Swiss hillshade effect—these are the same steps that the model goes through. You will need to use your original DEM and its default hillshade.
- Using Spatial Analyst’s Raster Calculator, enter the formula DEM / 5 + Hillshade. Then click Evaluate. This display simulates an areal perspective that makes the higher elevations lighter and the lower elevations darker.
- Now make the second raster. Using the Neighborhood Statistics Tool in Spatial Analyst, use the default hillshade as the input raster, set the statistic type to mean, the neighborhood to circle and the radius to 4 cells and click OK. The output grid generalizes the hillshaded terrain, emphasizing the major geographic features, minimizing the minor features, smoothing irregularities on the slopes, but maintaining the rugged characteristics of ridge tops and canyon bottoms. Now you can combine the grids in your display to get the final effect.
- Now symbolize the rasters:
- Move the original DEM to the top and symbolize it with a color ramp to show elevation – we’ll use a color ramp from the ESRI style that ranges from blue green to brown. Next, set the transparency to 55%.
- In the middle, display the neighborhood statistics grid with 35% transparency and use a single hue color ramp, like a black to white ramp.
- On the bottom, display the raster calculator grid with a black to white ramp and 35% transparency.
The final display produces an effect similar to the Swiss-style hillshade.
There is one other thing I want to mention which relates to the resolution of the DEM used for hillshading: There is an Ask a Cartographer question that deals with the issue of creating a higher resolution data set from courser resolution data, for the purposes of hillshading – take a moment to read it at: Improving the resolution of 90m SRTM data to 30m for use in cartography outside USA.
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