Working with offsets
Offsets are used for GIS data collection in the following situations:
- When mapping the location of an inaccessible feature because it is either difficult to reach or it is in not safe to reach. For example, a tree on an island, or a manhole in the middle of a busy road.
- When mapping the location of a feature where it is not possible to get a GPS signal, or a GPS position of sufficient accuracy. For example, under a large tree or in a narrow street or ‘urban canyon’.
Offsets combine the use of distance, bearing (or angle), and/or inclination (or slope) to determine the location of features. In ArcPad, there are two types of offsets: simple and complex.
A simple offset is performed on a polyline or polygon and only uses a distance measurement. Using a simple offset, you would map a feature that is a constant distance away from a polyline captured via pen or GPS. You would also specify whether the new feature is left or right of the captured polyline. For example, you could map the distance of a new sidewalk to the left of and at a distance of 5 meters from the center of an existing road.
Complex offsets can be used for point features and vertices of polylines and polygons. Complex offsets use a combination of distance, bearing, and inclination measurements. ArcPad supports 1 point offsets and 2 point offsets. A 1 point offset uses distance and bearing measurements (distance-bearing) to determine the offset. A 2 point offset uses either 2 distance measurements (distance-distance) or 2 bearing measurements (bearing-bearing) to calculate the offset. Inclination can be used in combination with any of these offsets, if a height value is known and you are storing your results in a shapefile that supports a z value.
1 point offset
A 1 point offset enables you to create a point or vertex offset from a single reference point. The reference point can be set using a pen, GPS, or manually entered coordinates. A 1 point offset uses bearing and two other parameters such as slope distance, horizontal distance, vertical distance, or inclination to calculate the offset. Refer to the diagrams below for an illustration of the distance and bearing parameters.
Typically you would select a 1 point offset if you were using a rangefinder that included a built-in compass. For example, you could capture a tree (target) that was a distance of 15m and had a bearing of 45 degrees from True North from your location, a known reference point.
2 point offsets
A 2 point offset enables you to create a point based on two reference points (A & B). The reference points can be set using a pen, GPS, or manually entered coordinates. There are two variations available in ArcPad for the 2 point offset: distance– distance and bearing–bearing. Typically, you would use a 2 point offset to calculate point features rather than vertices for a polyline or polygon.
A distance-distance offset is used when the bearings are not known from the reference points but the distances are. For example, when you are using a rangefinder without a compass. The distance between the feature and the two reference points A & B are used to calculate the location of the feature. The feature’s location is at the point where the two circles centered around reference points A and B intersect. Without a bearing, there are two possible solutions to this type of offset. You must, therefore, determine (and set) if the offset point is to the left or right of an imaginary line between Reference points A & B. Vertical distance or inclination are required if a correct 3-dimensional solution is to be calculated.
M or Measure field
The Offset and 2 Point Offset pages in the Point/Vertex dialog box include a box to enter a measure, or M, value for the target point. The x, y, and optionally z coordinates are calculated automatically but the M value needs to be entered manually. The M field is not a default field in a shapefile, and therefore must be created when the shapefile is created. Data can only be entered in this input box if the editable shapefile contains an M field.
A bearing–bearing offset is used when two bearings are known for the reference points but the distances are not. For example, when you are using a compass without a rangefinder. The bearing from north from each of the two reference points A & B is used to calculate the location of the feature. The feature’s location is at the point where the two bearing lines intersect. Vertical distance or inclination are required if a correct 3-dimensional solution is to be calculated.
By default, a 2 Point Offset in ArcPad is bearing–bearing. To activate the distance–distance solution, tap the bearing dropdown and set the location of the offset point. The distance– distance solution requires that you nominate which side of the A–B line the offset point lies. Once activated, the distance fields can be completed. With a rangefinder active, the fields are automatically populated.