Tag Archives: geodetic projection

Coordinate conversion from HK80 grid to Google Map and vice versa using Nspire

At work there is a need to deal with data from a GIS system and its own local coordinate system which is different from the usual one as in GPS and Google Map. Transformation between these two coordinate systems is possible and is well documented at this article (link). A practical application with Google Map and public CCTV images also required such conversion.

For casual usage, a handy transformation function is coded in the Nspire to convert XY values between the two systems. Since the calculation is pretty simple, the built in function script is enough for the task and as such there is no need to summon the power of lua.

web tool from the local authority was used to verify the results. Due to rounding or difference in calculation methods, a little discrepancy is observed but is well within tolerable margins.


Since we already have X,Y, why not utilize the nice color screen of the Nspire by adding a map with the result plotted on? GoogleMap API is not feasible as the Nspire is not geared for the Internet. If we can settle on something simpler, and forget zoom-in or switching to satellite view, a simple map in the background can do the trick as well. After-all, a full blown map software is not something this calculator excel at. But then, plotting the approximate location helps shed some lights on the true nature of the results from arguably its most used perspective – on the map.