Ticket #1970 (closed defect: fixed)

Opened 8 months ago

Last modified 8 months ago

Scale_Factor are rounded to 6 digits

Reported by: km5miel Assigned to: warmerdam
Priority: normal Milestone: 1.4.4
Component: OGR_SRS Version: unspecified
Severity: normal Keywords: proj4 scale factor precision
Cc:

Description

I’m using PROJ4 to project data from Swedish RT90 to Swedish SWEREF99. Although these are different datums, the national land survey recommend that one uses a projection on GRS 80 to go from RT90 to SWEREF99. The WKT:s that shall be used are:

From:

PROJCS["RT90_DIRPRJ",GEOGCS["Latitud/longitud.SWEREF99",DATUM["SWEREF99",SPHEROID["anon",6378137,298.257222101]],PRIMEM["Greenwich",0],UNIT["grader",0.0174532925199433]],PROJECTIONTransverse_Mercator?,PARAMETER["Central_Meridian",15.8062845294444],PARAMETER["False_Easting",1500064.274],PARAMETER["False_Northing",-667.711],PARAMETER["Latitude_of_Origin",0],PARAMETER["Scale_Factor",1.00000561024],UNIT["m",1]]

To:

PROJCS["SWEREF99_TM",GEOGCS["Latitud/longitud.SWEREF99",DATUM["SWEREF99",SPHEROID["anon",6378137,298.257222101]],PRIMEM["Greenwich",0],UNIT["grader",0.0174532925199433]],PROJECTIONTransverse_Mercator?,PARAMETER["Central_Meridian",15.0000000000000],PARAMETER["False_Easting",500000.0000],PARAMETER["False_Northing",0.000000],PARAMETER["Latitude_of_Origin",0],PARAMETER["Scale_Factor",0.99960000000],UNIT["m",1]]

Using PROJ4 I do however get a result that an approximately 3 meter from what I expect

One of the engineers at the Swedish land survey have looked at this and it’s likely that internal accuracy of the “Scale_Factor” are causing the problem. Changing the Scale_Factor from 1.00000561024 to 1.000006 gives about the same error in another more accurate software.

Change History

11/06/07 12:37:30 changed by warmerdam

  • keywords set to proj4 scale factor precision.
  • status changed from new to assigned.
  • component changed from default to OGR_SRS.
  • milestone set to 1.4.4.

A preliminary analysis suggests the problem is with OGRSpatialReference::ExportToProj?4() using %f for scale factors instead of the %.16g used for most other numbers. For instance, testepsg on the RT90 definition reports: 

+proj=tmerc +lat_0=0 +lon_0=15.8062845294444 +k=1.000006 +x_0=1500064.274 +y_0=-667.711 +ellps=GRS80 +units=m +no_defs

Note the truncation of the scalefactor.

11/06/07 13:15:23 changed by warmerdam

  • status changed from assigned to closed.
  • resolution set to fixed.

Fixed in trunk as r12624 and 1.4 branch as r12625. A regression test added in trunk as r12623.