For TAP 6.1.2513 or later systems with a Maintenance Subscription date of March 31, 2013 or later, you can run Map Feature Target Studies. Feature Targets are a new way to generate points for target studies. It allows the user to select map features, and then generates evenly-spaced target points along that feature (or feature boundary for polygons). This differs from the current “Shapefile” method, which simply uses the shape vertices, with no guarantee of equal spacing of points along the feature.
The following graphic shows Feature Target selection. To activate Feature Target selection, click the highlighted “Feature Targets” button in the “Coverage Area” frame. The yellow highlighted notification appears at the top of the map, informing you to click and drag the cursor to select a study area rectangle, which is labeled in the figure.
Upon releasing the left mouse button, the “Choose Map Layer Features” dialog appears. Note that a wait cursor will be shown because it may take some time for the dialog to appear as TAP determines the layers and features that intersect the selected area. In a feature-rich urban area, for example, the determination of intersecting features may not be instantaneous.
In the top right of the dialog, choose the units and value for the target point step, which is the distance between the generated target points.
The upper left shows the “Map Layers” grid, which is the list of all map layers that contain features that intersect the selection area. The first column contains a checkbox which allows you to select or deselect all features in that layer. The middle column is the layer name, and the last column shows the layer shape type. (Only the “Use All?” layer can be edited.) Click on any layer name to show that layer’s features in the “Layer Features” grid. In this instance, the “co_roads” layer is selected.
The bottom left grid is the “Layer Features” grid. This shows all the features in the currently selected layer. These features intersect the selection area, in whole or in part. The “Use?” column is the only editable column and allows the user to select or deselect that feature as a source of target points. The other columns list the properties of that feature, allowing the user to identify the individual feature(s) of interest.
After selecting the features and setting the target point step, click the “Continue” button to generate the target points. Note that a wait cursor will be shown while TAP generates the target points. The time will depend upon the step size and the number and length of the chosen features.
When the targets have been generated, control returns to TAP:
Notice the generated study points, highlighted above. A small black square is shown at every target point, which is also shown in the “Target Study” grid as usual. The target points are shown below in closer detail:
Feature Target Point Generation Algorithm:
TAP starts with the “first” point in the feature and performs the following algorithm.
- 1. Set Delta = target point step size.
- 2. If this point is in the study area, add it to the target list.
- 3. If the distance to the next feature point is less than the step size, then
- 3a. Delta = the difference between the step and the remaining distance.
- 4. Else
- 4a. Project this point by Delta along the azimuth to the next feature point.
- 4b. Delta = target point step size.
- 5. Go to step 2.
The following graphic shows the algorithm. The first target point (red “X”) is the step size from feature point 1. When the next point is projected from the target point along the azimuth between feature points 1 and 2, the next candidate point would be beyond point 2 and off the feature. So the difference is saved as “Delta”. The candidate point is re-projected along the azimuth between feature points 2 and 3, at a distance of “Delta” which is less than the target point step, thus generating the second target point “X”.
This algorithm generates the target points such that the spacing along the feature is consistently the target step size. It does not force feature vertices to be included in the points set.