This is a program that helps change from one co-ordinate system to another easily.
AllTrans can make worldwide professional coordinate transformations. Whether between international and national coordinate systems, with or without geodetic datum change, all can be handled with this program. The tool is quite versatile, as it can handle transformations between State-Plane-Coordinates (NAD83, NAD27, Transverse Mercator, Lambert, Soldner-Cassini, Mercator, EOV, Krovak, LV95, Pulkovo-Gauss-Krueger, Gauss-Boaga, etc.) and UTM-Coordinates, Geographical- and 3D-Cartesian-Coordinates as well. Inputs can be manual or you can import files ASCII, NMEA-GGA. You are able to export the same way. If the geodetic datum of source and the target systems are not the same then, you could do a datum shift too. For example, the alternatives available are 3-Parameter-Molodenski, 7-Parameter-Bursa-Wolf or Grid-Shift NTv2. The tool provides interfaces to Google earth and Google maps also.
For all the countries of the world, data is available for the shift to WGS 84. You could utilize this database if required. If you need better accuracies, then it is possible to calculate individual parameters using identical points on maps. The calculation can be done with a standard L2-adjustment or otherwise using an adjustment with re-weighting. The calculator implements a outlier test also. You could store these higher accuracy data for future use. If you were into orthometric corrections, you could use the Geoid-Modul (e.g. EGM2008, GEOID09) to derive the orthometric heights from GPS heights. The interface makes it easy by making everything you need, available on screen. However, that creates a bit of a clutter. Obviously, this is not for amateurs. You have to have a good understanding of the various transformations; else it will be easy to mess up.