Datum shift problem with user location

[GeorgeVernardos]

Our map service has

"spatialReference" : {
      "wkid" : 2100
    },

When we show the user location it is dislocated  (~150 m) from the real location.

Something in the conversion from Iphone WGS84 GPS DATA  --> GREEK GRID ( 2100 ) is wrong.


Here is a working projection that we use in our c# classes

PROJCS["GGRS87 / Greek Grid",GEOGCS["GGRS87",DATUM["Greek_Geodetic_Reference_System_1987",SPHEROID["GRS 1980",6378137,298.257222101,AUTHORITY["EPSG","7019"]],TOWGS84[-199.87,74.79,246.62,0,0,0,0],AUTHORITY["EPSG","6121"]],PRIMEM["Greenwich",0,AUTHORITY["EPSG","8901"]],UNIT["degree",0.01745329251994328,AUTHORITY["EPSG","9122"]],AUTHORITY["EPSG","4121"]],PROJECTION["Transverse_Mercator"],PARAMETER["latitude_of_origin",0],PARAMETER["central_meridian",24],PARAMETER["scale_factor",0.9996],PARAMETER["false_easting",500000],PARAMETER["false_northing",0],UNIT["metre",1,AUTHORITY["EPSG","9001"]],AUTHORITY["EPSG","2100"]]

[DiveshGoyal]

I'm assuming you're getting the location from the GPS. Bear in mind that the location reported by the GPS sensor is only approximate, depending upon the signal strength.

Can you reduce the problem down to what pair of WGS84 coordinates are being reported by the GPS, and what are the corresponding result coordinates in Greek Grid?

If you could share your code/project/service that might help.

_
Divesh

[GeoGuru]

You can accurately convert between GGRS87 (the Greek Grid) and WGS84 (longitude/latitude) in .NET using the toolkit at http://geodesy.codeplex.com.
If you want a direct algorithim check out this page (works in Javascript & C# as is and can be easily converted).