# To complete a resection

In a conventional survey, the resection function is used to perform a station setup and determine coordinates for an unknown point by making observations to known backsight points. The Trimble Access software uses a least‑squares algorithm to compute the resection.

A resection needs at least one of the following:

- Two angles and distance observations to different backsight points
- Three angles‑only observations to different backsight points

Because the resection calculation is a grid calculation, you can only use backsight points that can be viewed as grid coordinates. Do not compute a resection point and then change the coordinate system or perform a site calibration. If you do, the resection point will be inconsistent with the new coordinate system.

## To complete a resection

1. Tap and select Measure / <Style name> / Resection.
   1. When prompted, use the electronic level to level the instrument. Tap Accept.

   2. Set the corrections associated with the instrument.

      If the Corrections screen does not appear, tap Options and enter the correction information.

   3. Enter the instrument point name and the instrument height. See Station coordinates and instrument height.

   4. To compute the station elevation, select the Compute station elevation check box

      For a 2D or planimetric survey, clear the Compute station elevation check box. No elevations are calculated. To determine the elevation of a point with known 2D coordinates, once you have completed a station setup, see To determine the station elevation.

   5. Tap Options to configure the number of observations to make and the order in which to take them. Make sure the Face order setting is correct. You cannot change this setting after you start measuring points. See Station setup plus, resection, and rounds options.
   6. Tap Accept.
2. To measure the first point:

   1. Enter the first Point name and the Code, if required.

   2. By default the Backsight check box is selected.

      If you perform a resection or station setup plus while running an integrated survey, you can measure backsight points with GNSS. To do this, tap the Options softkey and then select Auto measure GNSS. Enter an unknown point name in the point name field. The software prompts you to measure the point with GNSS using the specified point name. The Measure softkey shows both a prism and a GNSS symbol. The Trimble Access software first measures the point with GNSS and then measures with the conventional instrument. Make sure that you have a site calibration loaded when combining conventional and GNSS measurements.

   3. Choose an option in the Method field.

   4. Enter the Target height.

      Make sure the target height and prism constant values are correct as you measure each point. You cannot change these values in subsequent rounds.

   5. Sight the target and then tap Measure.

      If you measure to static targets when there are two prisms close together, use FineLock or Long Range FineLock technology.

      If you are using a Trimble VX spatial station or Trimble S Series total station and the measurement is likely to be interrupted, for example, when measuring in traffic, select the Interrupted target measurement check box in the Target controls screen.

   The software shows the residual information for the observation.

3. Measure further points.

   To include foresight points during station setup plus, clear the Backsight check box. Foresight points do not contribute to the station setup result.

   In a conventional survey, when two measurements are completed, or when connected to a GNSS receiver or using a controller with internal GPS, the Trimble Access software can provide navigation information for further points. Tap Navigate to navigate to another point.

   When there is enough data for the software to calculate a resected position, the Resection residuals screen appears.

4. Use the information in the Residuals screen to review the quality of the observations and remove poor observations. See To review observation residuals and setup results.

5. To observe more points, tap +Point. Repeat steps 2 and 3 to add more points to the resection.
6. To take further measurements to points already measured (that is, measure rounds of observations):

1. Tap End face.

2. If you are using a servo or robotic instrument to measure a known (coordinated) point, tap Turn. Alternatively, to automatically turn a servo instrument to the point, set the Servo auto turn field in the survey style to HA & VA or HA only.

   When using servo or robotic instruments, check that the instrument has sighted the target accurately. When measuring a DR target with a Trimble total station with automated rounds, the software pauses to allow you to sight to the target. You must manually sight and measure the point to continue.

3. When the end of the rounds list is reached, if points have been skipped the software prompts you to return to observe the points that were skipped during that round. The observations can be skipped again if required.

7. When you have completed all observations, tap Results to view the resection results.
8. Tap Store.

You can use the resection function to perform an eccentric station setup, where the station setup is performed in view of a nearby control point and in view of at least one backsight point. For example, use this setup if you cannot set up over the control point, or you cannot see any backsight points from the control point. An eccentric station setup needs at least one angles and distance observation to a nearby control point and one angles‑only observation to a backsight point. Additional backsight points can be observed during an eccentric station setup. You can measure backsight points with angles‑only observations or with angles and distance observations.

## Helmert transformation for resection

When you enable the Advanced Geodetic check box in the Cogo settings screen, Resection has an additional computation method called Helmert transformation. To perform a resection using a Helmert transformation, select Options during a Resection, and then set the Resection type to Helmert.

The standard resection type is the same as the resection method used when Advanced Geodetic is not enabled.

For a Helmert transformation, you must measure distances to the backsight points. The resection calculation will not use a backsight point without a distance measurement.

For more information on Helmert transformations, refer to the Resection Computations in Trimble Access Reference Guide, which can be downloaded from the PDF guides page of the Trimble Access Help Portal.