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Depicting DEMs

July 13 2007 | 0 comments
Categories: Cartographic Effects, Data Modeling

I have a memory from a few UCs ago, where there was a presentation about rasters and cartography. This past UC brought them to mind and I hope you could explain 2 things. 1- the point was made about generalizing a DEM hillshade and using transparency to improve the display when it was combined with an ungeneralized hillshade. 2- The same presentation talked about how to use two gradients, ordered in perpendicular axes, to depict the effect of hillshading on e.g landcover. These two tricks have vexed me for years now and I'd like to know how they are done, and see if I am actually remembering correctly.

Any help would be greatly appreciated.

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I'm pretty sure what I'm thinking of is the presentation you gave at UC '05

Cartography: Advanced Techniques and Cartographic Realism abstract...

This session demonstrates cartographic enhancements to make maps that are evocative and eye-catching while remaining accurate and true to purpose. This can be achieved in part with cartographic realism by employing techniques that make some map features, such as terrain and natural landscapes, appear more realistic and therefore more intuitively understandable to a greater number of users. Techniques for enhancing cartographic realism demonstrated in this session include hillshading and elevation tinting, bump mapping, sun glints, coastal vignettes and stream tapering. Other advanced cartographic techniques include elevation tint legends, contour labeling and variable depth masking, drop shadows and polygon text elements. Together, the methods introduced here can be used to make maps that are often more attractive, even striking, compared to their conventional counterparts.

I'm going to assume that I'm thinking about the topic of elevation tinting as well as the Swiss hillshading.

I understand the topics conceptually, but the Arc execution escapes me. I would really like to be able to do this.

Again, thanks for the help and the quick reply.

Mapping Center Answer:

Do you mean the Swiss hillshading method (requires two hillshades (plus the original DEM for layer tinting) and the elevations tint legend (to create the legend for how the layer tint looks on top of the hillshade black-to-white color ramp)???

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If you want to step through the Swiss Hillshade method step-by-step so that you can follow the process, you can easily do so -- make sure you have the Spatial Analyst extension to follow the steps below:

To produce the Swiss effect, you will need to use your original DEM and its default hillshade. Here’s how you can create the default hillshade. In ArcToolbox, scroll down to the bottom to see the Spatial Analyst Tools. Click on the Surface tools and then double click Hillshade tool. Double click that to open it. Specify the digital elevation model that you want to use.

Now, using Spatial Analyst’s Raster Calculator, enter the formula:

DEM (this is the name of your DEM data file) / 5 + Hillshade (this is the name of the hillshade file you created in the last step)

Then click Evaluate.

This display simulates an aerial perspective that makes the higher elevations lighter and the lower elevations darker.

Now make the second raster. Click on the Spatial Analyst toolbar and select Neighborhood Statistics. 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.

To create the final display:

• Move the original DEM to above the two rasters we just created and symbolize it with a color ramp to show elevation – we have one we created especially for Crater Lake National Park. Next, set the transparency to 55%.

• In the middle, display the median filter grid with 55% 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 55% transparency.


The final display produces an effect similar to the Swiss-style hillshade.

Now to the elevation tint legend:

An Elevation Tint Legend shows the same symbology you used to make your map. The objective is to create a legend that contains the same transparency and color ramp specifications as any of the layers used to create your hillshaded–elevation tinted surface.

Say your map had a single hillshade overlain with a single elevation tint that had a transparency. Then you would need to create a legend using two layers displayed in the same order and with the same settings as the hillshade and the elevation tint on the map. For the Swiss Hillshade, you have three rasters instead of two.

First, you’ll want to create a rectangle feature in an otherwise empty polygon dataset. The shape should be similar to how you want the final legend to look. I often chose to make a long narrow horizontal rectangle.

Next, you would add a data frame that contains three copies of this rectangle data layer. Then you would use gradient fills to symbolize the rectangles using the same color ramps as on your map. For this elevation tint layer (the DEM layer with the colors), you would use a linear gradient fill with the elevation tint color ramp that you used on the map. On the Display tab and you need to set the transparency to the value you used on your map for the elevation tint.

Now look at the properties for the hillshade layers next. For the raster calculator layer, set the transparency the same as on the map. Now click on the Symbology tab and click on the symbol again. For this rectangle layer, use the same black to white ramp as the hillshade on your map and then be sure to change the angle to 90 degrees. Do the same for the neighborhood statistics layer. That’s it!

I hope this works for you!

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