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.

Linear traverse

A linear traverse enables you to capture the vertex of a polyline or polygon starting from a known point as you walk along a traverse. In other words, you begin at a known reference point, capture vertex A, move to vertex A, capture vertex B, move to vertex B, capture vertex C, and so on. Within a linear traverse, you use an automatic moving reference point, that is, the last vertex you captured.

You can capture vertices 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 on the Point/Vertex dialog box to set foresight or backsight.

Radial traverse

When you capture a radial traverse, you begin at a known reference point and capture the vertices around the perimeter of that reference point. You are always standing at the same fixed position while you are capturing multiple vertices. If your known reference point is a GPS location, variation in the GPS signal may impact your reference point accuracy.

The diagram below shows how a vertex in a line is calculated using a linear traverse versus a radial traverse, using one or two reference points.

Other options for offsets

When working with offsets, you can set other options or parameters on the Options page on the Point/Vertex dialog box or in the GPS preferences. These parameters are typically considered constants versus the parameters you would set to calculate individual offsets such as slope distance, vertical distance, horizontal distance, and inclination.

Constant parameters may be set at the beginning of a data collection task and only changed as needed. The following are set on the Options page on the Point/Vertex dialog box:

• Foresight—Shooting at your target from a known reference point.
• Backsight—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 the 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, the height stored with the feature is the height of the insulators. If you specify the target height, the captured feature (insulators) has 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 the GPS in relation to ground level. The GPS height is set on the GPS Preferences dialog box.