Local Plane Adjustments
To handle adjustments on a local plane with GeoLab, you can easily configure a map projection to represent the local plane. This can be done in two ways as follows:
- XY-Record Projection: This method makes use of the optional "XY-Record Projection" which you set (on the "Adjustment" tab) after clicking the "Edit default options" button on the Project/Setup tab, and you then use the XY record to specify your initial and control coordinates.
The two methods will produce the same results - the only difference between them is how you format your coordinates in your IOB file (NE vs. XY records).
The XY record specifies the local x (east/west plane coordinate, positive toward the east) and y (north/south plane coordinate, positive toward the north) coordinates for a point. The x-coordinate corresponds to the XY-Record Projection's easting coordinate, and the y-coordinate to the northing coordinate.
When performing local plane adjustments with GeoLab it is important to understand how things handled internally. The local plane used by GeoLab is actually a map projection, which allows you to use virtually any coordinates for the origin of the local plane (i.e. false easting and false northing), as well as allowing the plane to extend to relatively large areas without unduly distorting your network.
You must be careful to set the Origin Northing and Origin Easting values (in the Map Projections dialog) for the selected map projection, to values that are close to the coordinate values you will use for your network points. If these values are very different from the values of the local plane coordinates of your points, the effect will be that your points will be too far from the origin of the map projection. This would cause distortions because any map projection is only valid within a relatively small area. For example, if you use a UTM projection, the valid area is within a band of longitude that extends only three degrees of longitude on either side of the central meridian.
The Importance of Heights
Although you want to do a local plane adjustment, you must still be aware of the role that heights play in your network. The heights that GeoLab uses must be geometrically compatible with the measurements you enter.
GeoLab initializes a point's height to 0.0, and until you specify a height or elevation for a particular point (e.g. using an NEO or NEH record), that height remains set to 0.0. Explicitly setting only some of the heights in your network can especially cause large distortions because the other heights are still set to 0.0.
Probably the most important aspect regarding heights, is to ensure that they are geometrically compatible with your measured distances. GeoLab's DIST record is used to enter a slope distance. This means that, because GeoLab's DIST record is for a slope (marker to marker) distance measurement, the heights, HIs, and HTs, must result in the correct slope of the line the distance is measured along as well as the correct heights of the line's end points.
If you have actually measured horizontal distances, or if you have reduced your slope distances to horizontal distances, you can let GeoLab go ahead and use the initial heights of 0.0 (and enter the horizontal distances in DIST records).
If, however, you wish to enter slope distances, then you should enter the correct heights for all of your network's points.