Map Projections - Concepts & Links
If you want to learn more about map projections, here is a brief summary on UTM and WGS84 and some links for further reading:
UTM
UTM stands for Universal Transverse Mercator. It is a system of grids covering the entire earth, except for the polar regions. The system is based on north-south oriented "strips" following meridians with 6 degrees separation. Such a "strip" is denoted by a number. In the other direction, there are latitude bands, denoted by a letter. The number and letter together defines a UTM zone. (Often, an entire north-south "strip" is referred to as the UTM zone. This may, however, cause confusion concerning to which hemisphere a coordinate refers, hence the Grid Tool app requires the full UTM zone id, including the final letter.)
Note that UTM maps can be produced using different geodetic datums (i.e. different ellipsoids have been used as an approximation of the shape of the earth). When the Grid Tool app is used, it is important to check that the used map is based on WGS84. (NAD83 and ETRS89 are also OK since they differ only slightly from WGS84.)
There is a lot of information on the web about UTM, but it is hard to find a really good introduction text. There is an article on Wikipedia about UTM. In particular, check out the map over UTM zones.
WGS84
As noted above, WGS84 refers to a specific geodetic datum (see Wikipedia). To increase the common confusion further concerning the terminology, "WGS84" is often, in casual speech, used to describe the coordinate format that is used when reading latitude and longitude in degrees on a GPS device. The proper term for that coordinate format is actually "geodetic coordinates", but only few people are convenient with that term. For that reason, the terminology in the app and on this site is sometimes a bit sloppy. This is by intention, in order to not use a language that is too technical for most users. The meaning of the used terms should be obvious from the context.
National grids
Many countries are using their own national map projections. In the first version, Grid Tool is only supporting national grids for Sweden. Swedish coordinates can be given in two different systems: The old RT90 system (a.k.a. the Swedish Grid) and the new SWEREF 99 TM (a national adaptation of UTM-WGS84).
If you want to learn more about map projections, here is a brief summary on UTM and WGS84 and some links for further reading:
UTM
UTM stands for Universal Transverse Mercator. It is a system of grids covering the entire earth, except for the polar regions. The system is based on north-south oriented "strips" following meridians with 6 degrees separation. Such a "strip" is denoted by a number. In the other direction, there are latitude bands, denoted by a letter. The number and letter together defines a UTM zone. (Often, an entire north-south "strip" is referred to as the UTM zone. This may, however, cause confusion concerning to which hemisphere a coordinate refers, hence the Grid Tool app requires the full UTM zone id, including the final letter.)
Note that UTM maps can be produced using different geodetic datums (i.e. different ellipsoids have been used as an approximation of the shape of the earth). When the Grid Tool app is used, it is important to check that the used map is based on WGS84. (NAD83 and ETRS89 are also OK since they differ only slightly from WGS84.)
There is a lot of information on the web about UTM, but it is hard to find a really good introduction text. There is an article on Wikipedia about UTM. In particular, check out the map over UTM zones.
WGS84
As noted above, WGS84 refers to a specific geodetic datum (see Wikipedia). To increase the common confusion further concerning the terminology, "WGS84" is often, in casual speech, used to describe the coordinate format that is used when reading latitude and longitude in degrees on a GPS device. The proper term for that coordinate format is actually "geodetic coordinates", but only few people are convenient with that term. For that reason, the terminology in the app and on this site is sometimes a bit sloppy. This is by intention, in order to not use a language that is too technical for most users. The meaning of the used terms should be obvious from the context.
National grids
Many countries are using their own national map projections. In the first version, Grid Tool is only supporting national grids for Sweden. Swedish coordinates can be given in two different systems: The old RT90 system (a.k.a. the Swedish Grid) and the new SWEREF 99 TM (a national adaptation of UTM-WGS84).