SWEREF 99The SWEREF projections are map projections intended for detailed geodetic and cartographic uses within Sweden. There are two options available, SWEREF 99 TM which is commonly used for applications at the national level and SWEREF 99 18 00 which is the most accurate choice for mapping applications at the local level around Stockholm.
A more detailed description of SWEREF 99 choices is available here:
RT 90RT 90 2.5 gon V 0:-15 is a coordinate system that was previously used by Lantmäteriet. More information is available here:
WGS 84WGS 84 is a three dimensional reference system which was developed by the US authorities for use in positioning systems (GPS). Accuracy in WGS is often improved by combining data with for instance SWEREF 99.
Google Web Mercator (Web mercator)Google Web Mercator is a projection used by Google Maps, Bing Maps, and many other online mapping services. It has accuracy limitations which may need to be adjusted for in various usage scenarios. However, it is well supported and there are many libraries for its use.
A description is available here:
Here is an article that describes the pros and cons of using Google Web Mercator to project data on for instance Google Maps/Bing Maps:
Transformation of coordinatesThere is a useful tool for transformation of coordinates between systems available here: