9
Offsets
In this chapter:
■
■
Introduction
Generating offsets
9 Offsets
9.1
Introduction
A GPS antenna or conventional instrument target cannot always occupy a point directly, for example, if a point is the center of a tree or part of a building that is under a canopy. There are several ways to measure these points using the Trimble Survey Controller software.
You can use a conventional instrument and measure a point using an
offset method. For more information, see page 186. Alternatively you
can use a laser rangefinder, or calculate an offset from points in the database. This chapter shows you how to calculate offsets using the
Offset function in the Trimble Survey Controller software.
To do this, choose Offsets from the Survey menu in a real-time GPS survey or a conventional survey. Measure a topographic (topo) point or a Rapid™ point nearby, then make tape and/or azimuth measurements to the offset point. When you enter these measurements into the
Trimble Survey Controller software, it calculates the coordinates of the offset point and stores them in the database.
Remember to include any change in height details, as the height (or elevation) of the offset point is also calculated.
C
Warning – In general, do not compute offset points and then change the coordinate system or perform a calibration. If you do, these points will be inconsistent with the new coordinate system. An exception to this is offset points computed using the Brng-dist from a point method.
9.2
Generating Offsets
This section describes how to measure offsets.
To generate an offset point:
1.
Measure points near to the offset point.
2.
From the main menu, select Survey / Offsets.
3.
Select an option from the Method field and enter the required
information. For more information, see page 186.
1 8 4 Trimble Survey Controller User Guide
9.2.1
9.2.2
Offsets 9
4.
Tap È to calculate the solution(s). The Trimble Survey
Controller software calculates and displays the coordinates for the offset point. Tap s to store these in the database.
Note – If you use the Four point intersection method or the From a baseline method and then change the antenna height record for one of the source points, the coordinates of the offset point will not be updated.
Options Softkey
Tap o to display the Options screen.
Change the Distances setting and apply sea level correction if required.
Note – If the measured points were measured using GPS, the coordinates of the offset point can only be displayed as grid values if a projection and a datum transformation are defined.
If you are using a magnetic compass to provide azimuth measurements, specify the magnetic declination.
Azimuths/Angles
If necessary, define a south azimuth and change the incrementation direction of the grid coordinates. For more information, see South
Azimuth, and Grid Coordinates, page 64.
Azimuth and angle measurements can be referenced to grid 0°, true north, magnetic north, or the sun. To change this reference, access the
Azimuth origin field and select the required origin from the list:
• If you are working with magnetic azimuths, enter the local value for magnetic declination. For more information, see
Magnetic Declination, page 65.
Trimble Survey Controller User Guide 1 8 5
9 Offsets
9.2.3
• If you are working with sun angles, the Trimble Survey
Controller software uses GPS time and the current GPS position to calculate the position of the sun (the reference) at the time that the angle is entered.
Choosing a Method (Offsets)
The following sections describe each method for measuring offsets.
Note – You can only use the Bearing-distance from a point method during a postprocessed survey.
1 8 6 Trimble Survey Controller User Guide
Offsets 9
From a baseline
Use this method to calculate the coordinates of a point that is offset from a baseline defined by previously measured points. To do this:
1.
From the main menu, select Survey / Offsets.
2.
In the Method field, select From a baseline.
3.
As shown in the following diagram, enter the names of the offset point (8), start point (1), end point (2), right/left offset (3,
4), offset from end (5, 6), and a vertical distance from the end of the line.
Note – Enter a positive offset if the point is offset to the right (3) of the end point, or enter a negative offset if it is to the left (4).
Enter a positive offset if the point is outside (5) the end of the line, or enter a negative offset if it is inside (6) the line. Enter 0 for either the left/right offset or in/out offset if you do not want the point offset in that direction.
4.
Tap È to calculate and display the coordinates of the offset point (8). Tap s to store these in the database.
Figure 9.1 shows the screen that appears and a graphical
representation of this method.
4
6
In
2
5
Out
7
8
3
1
Figure 9.1
From a baseline
Trimble Survey Controller User Guide 1 8 7
9 Offsets
Four point intersection
Use this method to calculate the coordinates of an offset point at the intersection of two lines defined by four previously measured points.
To do this:
1.
From the main menu, select Survey / Offsets.
2.
In the Method field, select Four point intersection.
3.
As shown in the following diagram, enter the names of the offset point (5), start point (1) for line 1, end point (2) for line 1, start point (3) for line 2, end point (4) for line 2, and any vertical offset.
4.
Tap È to calculate and display the coordinates of the offset point (5). Tap s to store these in the database.
Figure 9.2 shows the screen that appears and a graphical
representation of this method.
2
4
5
3
1
Figure 9.2
Four point intersection
1 8 8 Trimble Survey Controller User Guide
Offsets 9
Note – The two lines defined by the previously measured four points do not have to intersect but they do have to converge at some point, as
shown in Figure 9.3.
Calculated point
3
2
4
1
Figure 9.3
Point calculated where lines converge
Trimble Survey Controller User Guide 1 8 9
9 Offsets
Bearing–distance from a point
Use this method to calculate the coordinates of an offset point at a certain azimuth and distance from a previously measured point. To do this:
1.
From the main menu, select Survey / Offsets.
2.
In the Method field, select Brng-dist from a point.
3.
As shown in the following diagram, enter the names of the offset point (4), and start point (1), the azimuth origin, the azimuth (2), the horizontal distance (3), and a vertical offset.
4.
Tap È to calculate and display the coordinates of the offset point (4). Tap s to store these in the database.
Figure 9.4 shows the screen that appears and a graphical
representation of this method.
4
1
2
3
Figure 9.4
Bearing-distance from a point
1 9 0 Trimble Survey Controller User Guide
