Geography Reference
In-Depth Information
Elevation Based on Massive Sets of Data: The Esri Terrain
Most computer-based models of elevation are based on relatively few measured points, usually with
considerable distance between them. With the development of airborne LiDAR, knowledge of surface
elevation has increased dramatically: Elevation points on the surface (x, y, and z) can be spaced horizontally
approximately two or three meters apart! GIS LiDAR (Light Detection and Ranging)
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datasets are collected
by aircraft using light from lasers that is bounced off the earth's surface and returns to the aircraft. The
time it takes for this round trip is precisely recorded and used to determine the distance from the aircraft to
the ground. Combined with other information, the elevation of the point on the ground can be calculated.
In the following exercise, you will add a LiDAR dataset to your map and look at a few of its properties.
LiDAR is not an image or a photograph but resembles both because, in addition to obtaining elevation
data, different surfaces return different amounts of the laser beam, so that objects or conditions can be
discerned from a LiDAR dataset.
15.
Start a new, blank map in ArcMap. From the [___]IGIS-Arc\River folder add as data the GPS
track. Click Add Data again and, in the same folder, you will find the File Geodatabase Terrain_
from_LIDAR.gdb. Double-click it. Add
KY_SP_North_NAD83\River_Bend_and_Water_Plant_Terrain
to the map. Zoom to the extent of this File Geodatabase terrain.
What you are seeing is a great number of triangles that indicate elevation, slope and aspect. The colors
of the triangles indicate elevation. However, because of the density of the data, you are not seeing all
the triangles that could be generated by the data. The creators of the LiDAR terrain representation rightly
assumed that, given the relatively small scale, you don't need, can't see, and can't use all the information
that a complete TIN of the area could display, so there is no reason to try to display a high level of detail.
This terrain is based on more than 440,000 points!
16.
Use the Identify tool to sample a few locations within the terrain. Write down a few elevations,
slopes and aspects:
Elevation
Slope
Aspect
17.
What does the scale text box in the Standard toolbar indicate? 1:__________.
Immediately under the name of the terrain layer in the T/C there is a designation preceded by
an asterisk (*).
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Contrast LiDAR with the more familiar concept of RADAR (Radio Detection And Ranging), developed around the
time of World War II, used to display aircraft positions and weather. RADAR uses radio waves in much the same way
as LiDAR uses light (laser) waves.
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