Making Effective Use of Shaded Relief
How to create shaded relief in GIS-based maps
Shaded relief, or hillshading, is a technique where a lighting effect is added to a map based on elevation variations within the landscape. It generally is intended to mimic the sun's effects - illumination, shading and shadows - on hills and canyons. This is a common tool for mapmakers and cartographers because people identify with it - peaks look like peaks, valleys look like valleys. This is a good way to get a (faux) 3-D effect without changing from a 2-D perspective. It often makes the difference between a (literally) flat, schematic-like map and a map that truly shows a picture of a landscape.
Shaded relief effect is derived from a digital elevation model (DEM), which is a raster dataset ofelevation values. DEMs and hillshades are separate datasets - a DEMcontains actual elevation values while a hillshade, though also araster, contains brightness values. These two data layers can be usedon their own or in combination.
Creating a Hillshade
Exact procedures for creating a hillshade will vary depending on your GIS software, so refer to your software's help menus for step-by-step guidance. However, there are a number of key points that are relevant to all hillshade creation.
Data sources: For the U.S., DEMs are commonly obtained from USGS and are available in a range of resolutions from10-meter (high resolution) to 90-meter (lower resolution) cell sizes. 30-meter data is available consistently across the conterminous U.S.,Hawaii, and Puerto Rico; 60-meter data is available for Alaska. 10-meter data availability varies by location - you can check the USGS site forcurrent availability. USGS also produces a global elevation model atapproximately 1-km resolution (30 arc second), as does NASA at variousresolutions through their SRTM (Shuttle Radar Topography Mission) dataset.
Resolution and scale: For generating a hillshade, use a DEM that is appropriate to your mapping extent and scale. A hillshade generated froma high-resolution DEM (say 10-meter) will not display well at smallscales such as 1:250,000, but may work well at 1:24,000. Conversely, ahillshade derived from a low-resolution (say 1-km) DEM will not work for large-scale mapping (e.g., 1:24,000) but may be ideal for1:1,000,000 scale mapping. You generally do not want to resample ahillshade, so better to work with the right resolution DEM from the beginning.
Units: Units are extremely important when creating hillshades. And thisdoesn't just mean your data's inherent x,y coordinate system units; thevertical (z) units are also critical. This is because the hillshadingprocess calculates slope and aspect based on each cell's x, y, and zvalues (location and elevation), so it is important that these units beconsistent. Each DEM should specify its horizontal units, likely built into the projection information - e.g., for a DEM that is in UTM Zone 10 NAD27 meters, meters specifies the horizontal (x, y) units. For DEMs, however, there is also a vertical (z) unit - each cell in the DEM specifies an elevation, but is that elevation in meters or feet? Once you attempt to calculate slope based on that elevation, the units become quite important. Check the metadata for the DEM elevation units, or, if unavailable, use common sense (is the highest peak in my area really 5,000 feet, or 5,000 meters - over 16,000 feet!).
Light source: Again, a hillshade specifies the brightness foreach cell on a map image. This brightness depends on how the cell ispositioned in relation to a single light source (e.g., the sun). Sowhere you position this light source will affect this brightness. There are two variables that are used to define the light's position - altitude and azimuth. Altitude ranges from 0 to 90 (degrees) and relates the angle the light source is from the horizon. 0 means the light source is on the horizon, 90 means the light source is directly overhead. Azimuth ranges from0 to 360 (also degrees) and describes where the light source ispositioned around the compass - north is 0, east is 90, south is 180,and west is 270. Play with these values until you find a combination that yields reasonable, legible, and attractive results (a good starting point is altitude=45, azimuth=315).
Using a Hillshade
Depending on the GIS software you use, how you incorporate a hillshade into your map may vary. Sometimes it is incorporated as a "lighting effect" or "brightness theme", other times it is added in as a transparent greyscale image that adds gradations of lightness or darkness to your other map layers. Check your specific software's help section for how best to add a hillshade layer.
Once the hillshade is part of your map, there are a number of effective ways it can be used to develop great background effects. For example, use your hillshade in conjunction with:
- a DEM - to create a nice colored elevation/relief effect
- contour lines - to create a real-world "topo" map
- an aerial image - to add texture and a 3-D perspective to your imagery
- your own project polygon layers - to add a terrific landscape dimension to properties or focus areas.
As always, keep in mind your basic cartographic principles when working with shaded relief. If too prominent, relief can detract from other information on the map. It also may not add value to a map of an area with little elevation change. Set your low and high hillshade values from dark grey to light grey (versus black to white) for a more subtle hillshade, or vary the transparency to emphasize or de-emphasize a relief effect.
from:http://landtrustgis.org/technology/advanced/shaded%20relief
