Two-dimensional systems - RT 90
The co-ordinate system that previously was used for map production at Lantmäteriet is RT 90. However, starting in 2007, RT90 was replaced by SWEREF 99 TM as the co-ordinate system for surveying and mapping at both Lantmäteriet and in many other governmental agencies. The Transverse Mercator projection is used for mapping. The projection parameters that were used in Sweden are given below. The official Swedish name of the grid system is Rikets nät and the full notation is RT 90 2.5 gon V 0:-15. The older co-ordinate system RT 38 was also used in conjunction with the Transverse Mercator projection as well as the regional systems (RT R). The latter co-ordinate systems are used with suitably chosen central meridians, see below.
The network on which RT 90 is based.
The field survey
The third national triangulation programme began in 1967 in the far south of Sweden and when it was completed in 1982 the whole of Sweden had been covered by a first-order network, with an average distance of about 30 km between stations, which was simultaneously densified with a second-order network in which the distance between stations was, on average, 10 km. Unlike classical triangulation, the terms first and second order represents a difference in side lengths and not a classification of accuracy. The field programme was based on trilateration methods: initially using MRA 101 Tellurometers which were later replaced by Model 8 laser Geodimeters. Of the 15 800 distances that were measured, 85 % were measured with Geodimeters. The majority of the measurements were made from specially designed lightweight towers which could be quickly built up to a maximum height of 30 m. In total, some 3 800 stations were determined with a relative length accuracy of 1-2 ppm (mm/km).
Adjustment
In order to make new co-ordinates available as quickly as possible, Sweden was divided in twelve regions, which were named as RT R 01 to RT R 12. When field work in a region was completed co-ordinates for the region were computed and made available. The computation programme began in the southernmost region and continued region by region northwards as the field work was completed. In the adjustments one station with co-ordinates in RT 38 or from one of the adjacent regions was held fixed. Angular observations from the old first-order triangulations and selected astronomic observations were included in the adjustments.
The adjustments were done using Bessel's ellipsoid 1841 with geoid heights in RAK 70. Horizontal co-ordinates were computed on the Transverse Mercator projection with local central meridians.
When all 12 regions had been completed a final total adjustment was carried out. The result is called RT 90.
Map projection parameters for RT 90 2.5 gon V 0:-15
| Parameter | Value |
|---|---|
| Reference ellipsoid | Bessel 1841 |
| Semi-major axis (a) | 6377397.155 |
| Inverse flattening (1/f) | 299.1528128 |
| Type of projection | Transverse Mercator |
| Central meridian | 15°48'29".8 E of Greenwich |
| Latitude of origin | 0° |
| Scale on the central meridian | 1.0 |
| False Northing | 0 m |
| False Easting | 1500 000 m |
In this system, x coordinates are measured northward from the Equator, which has an initial x value of 0 metres; y co-ordinates are measured from the central meridian, increasing eastwards, and to avoid negative values a false easting is used. The central meridian was originally defined as 2,5 gon west of Stockholm's old observatory but is today defined relative to Greenwich. (1 gon = 0.9 degrees.)
Projection zones in RT 90.
Projection zones, local central meridians
Mapping of Sweden in one projection zone would lead to projection errors of up to 2 m/km, which is not acceptable for large scale mapping and surveying (scale > 1:10 000). Sweden is therefore divided into six projection zones, each with its own central meridian. The distance between the central meridians is 2°15' (2.5 gon). The remainder of the projection parameters are the same as for 2.5 gon V, except that a local origin is used in some zones, see below. The limits of the zones are defined so that they follow administrative boundaries (dotted lines).
Central Meridians
7.5 gon V equals 11°18'29".8 east Gr.
5 gon V equals 13°33'29".8 east Gr.
2.5 gon V equals 15°48'29".8 east Gr.
0 gon equals 18°03'29".8 east Gr.
2.5 gon O equals 20°18'29".8 east Gr.
5 gon O equals 22°33'29".8 east Gr.
Local origin for co-ordinates
In Sweden a special notation is used for the presentation of the different false northings and false eastings which is given as an extension to the notation for the co-ordinate system. The system with different false northing and false easting was developed at a time when co-ordinates were mainly handled in analogue form. Today, when everything is in digital form, the system creates more problems than it once solved.
Examples of the notation system:
'0:-15' is the same as the standard case with
x0 = 0 and
y0 = 1 500 000 m
'61:-1' means
x0 = -6 100 000 and
y0 = 100 000 m
'61:-1' can also be written as
'61:14-15'
