Swedish grid

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Swedish grid (in Swedish Rikets Nät, RT90) is the coordinate system used for government maps in Sweden.

While negative numbers could be used to represent all four "quarters" of the earth (NE, NW, SE, and SW hemispheres) standard application of RT90 is only useful for the Northern half of the Eastern hemisphere where numbers are positive. The coordinate system is based on metric measures rooting from the crossing of the Prime Meridian and the Equator at 0,0. The Central Meridian used to be based on a meridian located at the old observatory in Stockholm, but today it is based on the Prime Meridian at Greenwich. The numbering systems first number represents the largest distance, followed by what can be seen as decimals. As such, X 65 is located half way between X 6 and X 7.

The coordinate grid is specified using two numbers, confusingly named X and Y but having inverted positions from mathematical graphs. X being the north - south axis and y the east - west axis.

Example: E1317000 N6620000 (or Y=1317000 X=6620000) denotes a position 183 km (1317km-1500 km) west of the Central Meridian and 6620 km north of the Equator, which happens to be somewhere near the town center of Arvika.

[edit] RT90 Map Projection Parameters

Parameter	Value
Reference Ellipsoid	Bessel 1841
Semi Major Axis	6377397.155 m
Inverse Flattening (1/f)	299.1528128
Type of Projection	Gauss-Krüger (Transverse Mercator)
Central Meridian:	E15°48'29.8"
Latitude of Origin	0°
Scale on Central Meridian	1.0
False Northing	0 m
False Easting	1500000 m