Working with traverses
Traverses in ArcPad enable you to create polylines and polygons using offsets. In ArcPad, there are two types of traverses: Linear Traverse and Radial Traverse.
A linear traverse enables you to capture the vertex of a polyline or polygon from a known point as you walk along a traverse. In other words, you begin at point A, capture point B, move to point B, capture point C, move to point C, and so on. With a linear traverse, you are always using a moving reference point, that is, the last point you captured.
You can capture points in a linear traverse using foresight or backsight. With foresight, you are standing at a known coordinate, such as reference point A, and you are shooting the target, such as the vertex of your feature. With backsight, you are standing at your target, such as the vertex of your feature, and you are shooting back to a known coordinate, such as reference point A. Use the Options page in the Point/Vertex dialog box to set foresight or backsight.
A radial traverse uses 1 or 2 fixed reference points and creates a traverse relative to those fixed points. In other words, you begin at point A and capture the points around the perimeter of A. You are always standing at the same fixed point while you are capturing multiple vertices. The diagram below shows how a vertex in a line is calculated using a linear traverses versus a radial traverse, using 1 or 2 reference points.
Other options for offsets
When working with offsets you can set other options or parameters on the Options page in the Point/Vertex dialog box or in the GPS Preferences. These parameters are typically considered constants versus the parameters that you would set to calculate individual offset such as slope distance, vertical distance, horizontal distance, and inclination.
Constant parameters might be set at the beginning of a data collection task and only changed as needed. The following are set on the Options page of the Point/Vertex dialog box:
- Foresight: Looking forward to and shooting at your target from a known reference point.
- Backsight: Looking back at and shooting from your target to a known reference point.
- North Reference: Set your bearing to be calculated from True north or Magnetic declination read from a GPS (if available).
- Observer Height: Height of rangefinder in relation to a reference point at ground level.
- Target Height: Height of target above ground. By default, the target height
is 0 or unknown. If the target height is known and specified, the resulting
coordinate will be the XYZ value of the feature at ground level; the target
height is subtracted. If the target height is not known (value of 0), the
resulting coordinate will be the XYZ value of the actual target.
For example, if you are capturing insulators on power poles that always have a fixed height, you have two options. If you do not specify a target height, then the height stored with the feature is the height of the insulators. If you specify the target height, the captured feature (insulators) would have the coordinates of the power pole at ground level.
In another scenario where you are capturing treetop heights, the target height would be set to 0 since the height of the trees is always changing. The captured heights of the treetops would therefore be the height of the treetops above ground.
- GPS Height: Height of GPS in relation to ground level. The GPS Height is set in the GPS Preferences dialog box.