# Instrument corrections

You can set the corrections associated with conventional observations. By default the Corrections screen appears automatically after the Electronic level screen when you start a survey.

If the Corrections screen does not appear, tap Options and enter the correction information. To reset the default setting so that the Corrections screen appears automatically, tap Options and then select the Show corrections on startup check box.

If you intend to perform a network adjustment in the software using data from a conventional survey, make sure that you enter a pressure, temperature and, curvature and refraction correction.

Use the PPM (Parts Per Million) field to specify a PPM correction to be applied to electronic distance measurements. Key in the PPM correction, or enter the pressure and temperature of the surrounding environment and let the software compute the correction.

Typical pressure ranges are between 500 mbar–1200 mbar, but when you work in an area with over‑pressure (for example, a tunnel), larger pressures up to 3500 mbar are possible.

If you are using a that has an internal pressure sensor, the pressure field is set automatically from the sensor in the instrument. To disable this, tap the advanced pop‑up arrow and then clear the From instrument check box.

Use the Curvature and Refraction fields to control curvature and refraction corrections. The earth curvature and refraction corrections are applied to vertical angle observations and therefore have an impact on computed vertical distance values. They also affect the horizontal distance values to a very small extent.

The earth curvature and refraction corrections can be applied independently using the options provided. The earth curvature correction is the most significant correction with a magnitude of approximately 16" per km measured distance (subtracted from the zenith vertical angle).

The magnitude of the refraction correction is affected by the refraction coefficient, which is an estimate of the change in air density along the light path from the instrument to the target. Since this change in air density is affected by factors such as temperature, ground conditions, and the height of the light path over ground, it is very difficult to determine exactly which refraction coefficient to use. If you use typical refraction coefficients such as 0.13, 0.142, or 0.2, the refraction correction results in a correction in the opposite direction to the earth curvature correction with a magnitude of approximately one‑seventh of the earth curvature correction.

- The DC file format only supports a curvature and refraction correction that are both off or both on. When they are both on, they have a coefficient of 0.142 or 0.2. When settings other than these are used in the software, the settings exported to the DC file will be a best match.
- Do not set corrections in both devices. To set them in the software, make sure the instrument settings are null.

For some instruments, the software automatically checks to see if various corrections (PPM, prism constant, and curvature and refraction) are being applied correctly. If it finds that the corrections are being applied twice, a warning message appears.

In the following table, the * symbol in a field indicates that the correction at the top of that column is applied. The *' symbol applies only to computed coordinates when a station setup has been defined. For an explanation of the correction types, see the definitions below the table.

Displayed/Stored data | Corrections applied
--- | ---
C/R | PPM | PC | SL | Orient | Inst ht | Tar ht | Proj Cor | Stn SF | NA | POC
Status line | – | – | – | – | – | – | – | – | – | – | –
HA VA SD (raw) | – | – | – | – | – | – | – | – | – | – | –
HA VA SD | * | * | * | – | – | – | – | – | – | – | *
Az VA SD | * | * | * | – | * | – | – | – | – | – | *
Az HD VD | * | * | * | – | * | * | * | * | * | – | *
HA HD VD | * | * | * | – | – | * | * | * | * | – | *
Grid | * | * | * | * | * | * | * | * | * | * | *
delta Grid | * | * | * | * | * | * | * | * | * | * | *
Station and offset | * | * | * | * | * | * | * | * | * | * | *
DC file (observations) | – | – | – | – | – | – | – | – | – | – | *
DC file (reduced coordinates) | * | * | * | * | * | * | * | * | * | * | *
JobXML (observations) | – | – | – | – | – | – | – | – | – | – | *
JobXML (reduced coordinates) | * | * | * | * | * | * | * | * | * | * | *
Survey Basic | * | * | * | *' | * | * | * | *' | *' | *' | *

## Correction types

C/R

Curvature and/or Refraction correction.

PPM

Atmospheric parts per million (PPM) correction. PPM is calculated from temperature and pressure.

PC

Prism constant correction.

SL

Sea level (ellipsoid) correction. This correction is applied only if a fully defined coordinate system definition is being used; the correction is not applied in the Scale factor only definition.

Orient

Orientation correction.

Inst ht

Instrument height correction.

Tar ht

Target height correction.

Proj Cor

Projection correction. This includes the application of a scale factor specified in the Scale factor only definition.

Stn SF

Station setup scale factor. In any station setup, a scale factor for this setup can be specified or computed. This scale factor is applied in the reduction of all observations from this station setup.

NA

Neighborhood adjustment. In a station setup defined using station setup plus or resection, a neighborhood adjustment can be applied. The neighborhood adjustment is computed based on the observed residuals to the control points used during the station setup. The adjustment is applied, using the specified exponent value, in the reduction of all observations from this station setup.

POC

Prism offset correction. This is only applied when using a Trimble 360° prism, VX/S Series MultiTrack prism, VX/S Series 360° prism, R10 360° prism, Active Track 360 target, or Trimble Precise Active target.