Basic Feature Identification Interpreting Different Scales Interpreting Change GIS Tools for LIDAR Data

In the sections below, there are several different views of LIDAR data in and around the City of Westport, Washington, and Cape Meares, Oregon (see image at left). Pseudo color images are commonly used as an effective means of visualizing data. The term "pseudo color" refers to the fact that these images are not photographs. Rather, they are digital images in which each image pixel represents a data point that is colored according to its value. In the case of LIDAR data, the value represents an elevation measurement. The purpose of this section is to aid users in interpreting LIDAR pseudo color images.

Basic Feature Identification

The following animation is designed to point out basic features visible within a LIDAR grid.  The first step in viewing a "pseudo color" image is to check the color bar associated with the image.  The color bar indicates the parameter being displayed and the range of variable values represented by each color shade.  The color bar below shows elevations; the range of elevations for the region shown is between -4 and greater than 25 meters.  The interval represented by each color shade is 1 meter.

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Interpreting Different Scales

One can use the LIDAR Data Handler to manipulate LIDAR data sets for analysis and information extraction. Here, the "Legend Scaler" has been used to scale two images of Cape Meares, Oregon, differently to bring out different features. In the image on the left below, the scale has been set such that the features on the beach (blue to orange) are more prominent. Looking at the scale bar to the right of the image, we see all elevations greater than 48 meters are assigned a single color (brown).  This emphasizes features at lower elevations due to the smaller interval (2 meters) between the different colors available in the color bar.  The image on the right shows an enlarged version of the cape (indicated by the black rectangle in the left most image), scaled so that the elevations from 400 meters to 825 meters are enhanced.  The detail along the shoreline visible in the image to the left is lost, but one is now able to resolve features at higher elevations.  Note the interval on the color bar is now 30 meters for the rightmost image.  The areas of white within the images indicate no data were collected over those regions.

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Interpreting Change

Horizontal Change The animation below shows the erosion of the shoreline along Cape Shoalwater, Washington. The net result along most of this stretch was a landward retreat of the shoreline between 1997 and 1998. Notice also the slight accretion in the area where the shoreline begins to turn to the north.

Vertical Change The image below shows the scale and spatial distribution of the vertical elevation change from 1997 to 1998.  The region shown is identical to that pictured in the animation above. Using the color bar, one sees that yellow and red indicate erosion and green indicates accretion.  Only change greater than 0.5 meters is shown.  The change map is derived by subtracting the 1998 elevation grid from the 1997 elevation grid.  The "Grid Subtractor" option on the LIDAR Data Handler utility provides a simple means to produce a change map (grid).

Using Profiles The LIDAR Data Handler provides a tool for generating beach profiles from a LIDAR grid or grids.  Profiles from multi-temporal surveys may be used to assess the horizontal and vertical beach change at a specific beach location.  The profiles also provide a method to assist in locating the beach/water interface at the time of the survey.  The profiles below show 140 meters of horizontal beach retreat and up to 3 meters of elevation change between the 1997 and 1998 LIDAR surveys for a narrow section of the beach in the Cape Shoalwater grids pictured above.

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