There are two major kinds of coordinate systems:

Geographic Coordinate System (GCS): a 3D reference system that uses latitude and longitude to define the location of a point on the Earth’s surface (spheroid);
Projected Coordinate System (PCS): cartesian coordinates (Easting/Northing) to represent the location of a point on a two-dimensional surface. They are created by projecting the 3D GCS onto a two-dimensional surface.

Because depending on the kind of computations that have to be done it might be more efficient to rely on GCS or PCS coordinates, MeteoIO transparently computes both sets of coordinates from what has been provided as input. However, in order to do so it is necessary to provide the input Projected Coordinate Systems (if coordinates are given as PCS) or a suitable PCS (if coordinates are provided as lat/lon). The same applies to the output where the desired coordinate system must also be specified for the outputs (in the output section). This is done through the use of the COORDIN and COORDPARAM keys (see the documentation for each plugin).

There are two ways of supporting a given coordinate system: through the use of an adhoc implementation (that becomes part of MeteoIO) or through the use of an external library, Proj [ref: http://trac.osgeo.org/proj/]. The current internal implementations are the following (given by their keyword):

CH1903 or CH1903+ for coordinates in the Swiss Grid [ref: https://backend.swisstopo.admin.ch/fileservice/sdweb-docs-prod-swisstopoch-files/files/2023/11/14/7f7bf15b-22e2-48b6-b1ab-6905f81dca8a.pdf] (epsg codes, respectively 21781 and 2056)
UTM for UTM coordinates, the zone must be specified in the parameters, for example 31T [ref: http://www.oc.nps.edu/oc2902w/maps/utmups.pdf] (epsg codes as 32600+zoneNumber in the northern hemisphere or as 32700+zoneNumber in the southern hemisphere)
UPS for Universal Polar Stereographic coordinates (the zone, either N or S, must be specified in the parameters). [ref: J. Hager, J. Behensky, B. Drew, THE UNIVERSAL GRIDS: Universal Transverse Mercator (UTM) and Universal Polar Stereographic (UPS), 1989, Defense Mapping Agency, DMATM 8358.2] (epsg codes as 32661 for the north pole and 32761 for the south pole)
LOCAL for local coordinate system (using the horizontal and vertical distance from a reference point, see Coords::geo_distances for the available choice of distance algorithms)
PROJ for relying on the Proj library for handling the coordinate conversion. The EPSG code is provided as COORDPARAM (for example 21781 is the EPSG code for the CH1903 coordinate system). The maximum standard value for such a code is 32766.
PROJ_STR for relying on the Proj library for handling the coordinate conversion and providing a full definition string as COORDPARAM argument. For example, "+proj=longlat +ellps=bessel +towgs84=674.374,15.056,405.346,0,0,0,0 +no_defs +type=crs" for the CH1903 coordinate system.

Such an example of use is the following:

COORDSYS = UTM

COORDPARAM = 31T

Relying on an EPSG code with the PROJ library:

COORDSYS = PROJ

COORDPARAM = 21781