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.

Auto Resection

The resection workflow has built-in Auto Resection functionality, which enables you to calculate a resection without needing to know the name of the point at which the target is located. This can make performing a resection much faster on a busy site with multiple targets set up on permanent control points.

To use the Auto Resection function, leave the Point name field empty when measuring to the target. The Auto Resection routine in Trimble Access software automatically matches the observation with a point in the job that is a potential good fit. If you measure to a target that is located to a position not in the job or in a linked file then the measurement is automatically excluded from the resection calculation.

If multiple potential solutions are found, you can use the Solutions softkey to cycle through the solutions and review them in the map alongside the Resection - Residuals form. If required you can change the selected point to recalculate the resection, or measure to another point.

To be able to use the Auto Resection function, make sure the names of the points are included in the job, or in a linked CSV, TXT, or job file. Continuous topo points, construction points, scan points, and rapid points cannot be used for Auto Resection.

Watch the video Auto resection in Trimble Access on the Trimble Access YouTube channel to learn more about completing a resection using the Auto resection function.

To complete a resection

  1. If you plan to use the Auto Resection function, make sure the names of the points are included in the job, or in a linked CSV, TXT, or job file.
  2. 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.
  3. To measure the first point in the resection:

    1. If you know the Point name, you can enter it or select it.

      To perform an auto resection, where the software automatically locates the point, leave the Point name field empty.

    2. Enter or select the Code for the point, if required.

    3. By default the Foresight only check box is not selected.

      To store the point as a foresight observation, select the Foresight only check box. Foresight points do not contribute to the resection calculation.

      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.

    4. Choose an option in the Method field.

      If you select the Angles only or the H. Angle only method, you must enter or select a point in the Point name field. The point name is required whether you are completing a resection or an auto resection.

    5. 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.

    6. 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.

  4. Measure further points.

    1. Leave the Point name field empty to perform an auto resection and allow the software to automatically locate the point.

    2. Select the Code, measurement Method, and enter the Target height for the point.

    3. To include foresight points during the resection, select the Foresight only check box. Foresight points do not contribute to the resection 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.

  5. Use the information in the Resection - Residuals screen to review the quality of the observations and remove poor observations.

    An asterisk (*) next to the point name in the Resection - Residuals screen indicates that the point name was not entered during the resection, and instead the observation was automatically matched by the Auto Resection routine in Trimble Access as a potential good fit.

    If multiple potential solutions are found, a warning message appears in the software and the Solution softkey indicates the number of possible solutions found, for example "Solution 1 of 4". The "best" solution is the first one in the list. To reduce the number of solutions, measure to another point, or select the correct solution before storing.

    To select the solution you want to use, tap the Solution softkey to cycle through and view all solution residuals. The selected resection solution is shown in the map alongside the Resection - Residuals screen. Yellow lines indicate observations, and the solid yellow circle is the calculated station. Select the point to use in the Resection - Residuals screen and then tap Accept.

    If you are not sure which of the current solutions is correct, or if you are not sure any of them are correct, then you can do one of the following:

    • If you know the name of one of the points, highlight one of the observations and then tap Details and enter the name of the point.

    • If you do not know the names of any of the points, tap + Point to add another observation.

    For more information, see To review observation residuals and setup results.

  6. To observe more points, tap +Point. Repeat steps 3 and 4 to add more points to the resection.
  7. 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.
  8. When you have completed all observations, tap Results to view the resection results.
  9. 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.