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Accuracy
The accuracy of GPS positions used to be affected by an intentional degradation of the signal (SA). This degradation no longer occurs, but the quality of the receiver itself and the physical surveying conditions can influence the results. Under normal conditions the error can be in the order of 5-15 metres. To achieve higher accuracy differential GPS methods must be used, for example in Sweden by applying corrections obtained from Lantmäteriet's SWEPOS stations (through SWEPOS Network DGPS Service or Teracom's EPOS service). The corrections are received through a GSM/GPRS connection or an RDS receiver that is connected to the GPS receiver. You can read more about GPS on the SWEPOS page.
GPS and Swedish maps
The Swedish national map series used to be based on the co-ordinate system RT 90 2.5 gon V, but are since January 2007 based on the reference system SWEREF 99 and the projection SWEREF 99 TM. When GPS positions (in WGS 84) are transformed to any of these projections other errors occur.
SWEREF 99 TM
To obtain co-ordinates in SWEREF 99 TM, the correct parameters for a Transverse Mercator projection must be used. The difference between SWEREF 99 and WGS 84 is (at present) neglectible in this context. The parameter values to be used are:
Parameter Value
Central meridian 15° E Greenwich
Scale reduction factor 0.9996
False northing 0 m
False easting 500 000 m
The parameter values are the same as for UTM projection zone 33, but if the GPS receiver automatically changes zones the setting 'UTM zone 33' can only be used within the zone.
RT 90 2.5 gon V
Another error occurs when the GPS positions (in WGS 84) are transformed to RT 90. Hand-held GPS receivers (Garmin and others) utilize a 5-parameter transformation. This gives a less accurate result than the offically published transformation, which is a so-called direct projection. The quality of the transformation can be improved by using a special User grid or Local XYZ differences instead of RT 90 and the Swedish grid. These two alternatives are presented below.
Improved transformation to RT 90 with User grid
By using a specially produced projection, instead of RT 90 and the Swedish grid, the transformation can be improved for, for example, Garmin GPS 12. This is done using a so-called User Grid based on a Transverse Mercator projection. The grid as it appears in the receiver's display is shown in the illustration below:
Parameters for User Grid
Note:
The MAP DATUM used for the User grid must be WGS 84! (If Latitude of Origin is requested, it should be given as 0°)
These settings are applicable for RT 90 2.5 gon V, which is the reference system used for Swedish maps. There are approximate transformations for systems other than 2.5 gon V (pdf).
Improved transformation to RT 90 with Local XYZ differences
Some GPS receivers, such as Magellan, do not accept negative False Northing values. In such cases a locally improved transformation can be carried out by determining a datum shift for a point in the area and then applying it as a User datum.
This User datum must be combined with a User grid, equivalent to the RT 90 grid (not the User grid described above).
The procedure is explained below:
USER DATUM, Local to WGS 84
(Local in this context means RT 90 and is usually defined in the GPS receiver as below)
User Datum
Calculate values for Delta-X, Delta-Y and Delta-Z for a point, which should be located in the centre of the geographic area for which the transformation is to be applied. Latitude and longitude values can be taken from the map. The error of this type of local transformation will normally be less than one metre for an area of several tens of kilometres. See the illustration at the bottom of this page.
USER GRID equivalent to RT 90 2.5 gon V (for Swedish maps)
User Grid (RT 90 2.5 gon V)
To obtain values for Delta-X, Delta-Y and Delta-Z the Co-ordinate Transformation Online, via the link to the right, can be used with the option:
SWEREF 99 lat long -> RT90-SWEREF99-XYZDIFF
The indata is latitude and longitude and the outdata will be Delta-X, Delta-Y and Delta-Z values that can be used in the User datum above.
