topic Re: Datum shift problem with user location in ArcGIS Runtime SDK for iOS Questions

Datum shift problem with user location

GeorgeVernardos — Tue, 11 May 2010 14:50:51 GMT

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"]]

Re: Datum shift problem with user location

DiveshGoyal — Thu, 13 May 2010 16:15:15 GMT

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

Re: Datum shift problem with user location

GeoGuru — Wed, 22 Feb 2012 09:58:04 GMT

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).