When the data are downloaded from the survey instrument to a computer, mostly through a serial/USB cable, the measurement data are saved together with other information about the survey, like time of survey, type of instrument, direction of traverse, zero drift, zigzag/parallel survey orientation etc. These form some of the survey’s metadata (see Data documentation and metadata). The data and metadata are usually stored in separate computer files, often in a proprietary format designed by the manufacturer of the hard- or software.

Sequential

For instruments that store data as a long list of values on the survey instrument, the following steps are required during the data download.

• The download software needs to know the size of data grids. This information can either be entered by the user who knows in what size the data were originally collected (e.g. for Geoscan instruments) or by the instrument first sending this information to the downloading software to make it aware of it (e.g. for Bartington instruments).
• A stream of data is transferred to the computer, usually via a serial cable, when the ‘Dump’ procedure is initiated.
• On the computer, and usually after the full download, this stream is broken into appropriate data grids which are stored as smaller lists of data in individual files and related information (e.g. size of the data grids and their dimensions) are stored in metadata files to correlate each value in a data grid’s file with the correct grid position.
• For line data (e.g. from a Scintrex or Geometrics magnetometer) the marker information is evaluated (e.g. “every 5 m”) and the location for each recording is estimated and stored with the measurement value.

Location tagged

If the location data are already stored in the geophysical instrument during the survey, the procedure is somewhat simpler.

• Measurement data and positions are transferred to the computer together.
• These data are stored in an unsorted list of coordinates and values, often referred to as ‘XYZ text file’; this can also be in the form of a text file in which each line contains one data value, the survey line number, the reading number along the line, a marker indicator and a time stamp.
• The data are not broken into data grids at this stage.

If the position information comes from a non-integrated device (e.g. an auto-tracking Total Station), the data and position information have to be merged using the instruments’ respective timestamp information. It is obviously important to know which coordinate system or datum is used for these coordinates.