You could also use the SpatialReferenceConverter from ESRI.ArcGIS.Mobile.SpatialReferences to convert between any two coordinate systems that ESRI support, e.g. SpatialReferenceConverter src = new SpatialReferenceConverter(
SpatialReference.CreateWgs84SpatialReference(),
SpatialReference.Create(2264));
Coordinate xy = src.FromSpatialReference1ToSpatialReference2(new Coordinate(lng, lat)); To convert from WGS84 to NAD_1983_StatePlane_North_Carolina_FIPS_3200_Feet.
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