This tool allows a user to filter out unnecessary points in cross sections. With the use of GIS data, cross sections can contain many more points than actually necessary to describe the terrain. HEC-RAS has a limit of 500 points in any cross section. Because of this limit, it is occasionally necessary to filter out points that are not needed. To bring up this editor, select Cross Section Points Filter from the Tools menu of the geometric data editor. When this option is selected a window will appear as shown in Figure 5-87.
As shown in Figure 5-87, the editor allows the user to filter points on a cross section by cross section basis, or for a range of cross sections at one time (Multiple Locations option tab). To filter a single cross section, the user selects the river, reach, and river station they want to work on. Then press the button labeled Filter Points on Selected XS to filter the points.
Figure 5 87 Cross Section Points Filter Editor
The cross section points filter performs two different types of filtering on each cross section. The first type is called a Near and Colinear Points Filter, this method simply searches for points that are close together. If two points are found to be within the horizontal and vertical distance tolerance, then the second point is removed. The second pass of this filter searches for points that are in a straight line, or nearly in a straight line. This filter searches to find three consecutive points that may be in a straight line. If a line is connected between points one and three, and point two is less than a predefined tolerance from that line (vertical filter tolerance based on a distance perpendicular to the line), then the second point is a candidate to be removed. A second check is done to ensure the slope of the line that connects point one and two together, is not changing significantly when point one and three are connected (minimum change in slope tolerance). Options are available to only filter cross sections that have more than 500 points, as well as to restore a cross section back to the original points before filtering occurred.
The second type of filter is called Minimize Area Change. To use this filter press the Minimize Area Change tab below the graphic window. When this tab is selected the user will be asked to enter the number of points that they would like the new cross section to be trimmed down to. After a number is entered, the user presses the Filter Points on Selected XS button to perform the filtering. This filtering method will drop out one point at a time until the cross section is down to the user desired number of points. The decision process for dropping a point is to find the point in the cross section that will cause the area of the cross section to change the least.
Additionally, this editor allows the user to select multiple cross sections and perform the filter operation on all of them at once. This is done by first selecting the Multiple Locations tab. Then select the cross sections that you would like to filter. Set the filter tolerances to any desired values, and then press the Filter Points on Selected XS button.