Observation files define the survey observations that SNAP analyses to test the quality of the observations and to derive new coordinates for the survey marks.
SNAP handles most data types used in surveying. Generally SNAP uses the data as it is measured (the exceptions are projection bearings and sea level distances). The input data should not be reduced to a projection. Only environmental corrections (such as atmospheric temperature corrections to distance data) should be applied. For GPS data the GPS reduction should be done to derive baseline or multistation vectors or point positions.
Observations are entered as measured from an instrument to a target. The instrument and target are assumed to be located at a specified height above their survey stations. SNAP has no facility for handling eccentric observations. To use eccentric data you must either correct the data for the eccentricity or treat the eccentric location as a survey station distinct from the central station.
SNAP supports three types of observation file - SNAP, CSV, and SINEX formats. The SNAP format is more flexible in terms of handling all the data types and options supported by SNAP. The CSV format is more limited, but more suitable for integration with other programs. The main limitations of the CSV format are that it doesn't handle correlated sets of GPS baselines or point positions. The CSV format also requires a format definition file to specify the organisation of the observations in the file. The SINEX format is used by GNSS processing software to output the results of processing GNSS data. It is limited to point data only.
SNAP can use multiple data files files each of which can contain many types of data - the data files are defined using data file commands in the SNAP command file.
For SNAP format files SNAP uses units of metres for all distances, and degrees, minutes, and seconds for all angles. Angle errors and residuals and reference frame rotations are expressed in seconds. Scale errors are measured in parts per million (ppm).
For CSV formatted files all distances and distance errors are expressed in metres, and all angles and angle errors are expressed in degrees.