[SIS-547] Mercator projection should wraparound longitude values

• Type: Improvement
• Status: Closed
• Priority: Major
• Resolution: Fixed
• Affects Version/s: 0.6, 0.7, 0.8, 1.0, 1.1, 1.2
• Fix Version/s: 1.3
• Component/s: Referencing
• Assignee: Martin Desruisseaux
• Reporter: Martin Desruisseaux
• Created: 11/May/22 16:36
• Updated: 03/Oct/23 08:41
• Resolved: 20/Aug/22 14:28

Description

When the central meridian has a value different than zero, user may expect a range of 180° on the east of central meridian to produce positive easting values and conversely on for 180° on the west of central meridian. This is currently not the case:

ProjectedCRS crs = (ProjectedCRS) CRS.forCode("EPSG:3994");
MathTransform prj = crs.getConversionFromBase().getMathTransform();
double[] coordinates = {-41, 100, -41, 179, -41, 181, -41, -179};
prj.transform(coordinates, 0, coordinates, 0, 4);
System.out.println(java.util.Arrays.toString(coordinates));

Output (reformatted for readability):

        0   -3767132
  6646680   -3767132
  6814950   -3767132
-23473717   -3767132

Other map projection implementations rely on trigonometric functions for applying an implicit wraparound. For example in a call to sin(λ) the λ argument value is implicitly reduced to a range of -π … +π around the λ₀ (central meridian). But it does not happen in the particular case of the Mercator projection, since the Easting value is just a multiplication factor without trigonometric functions involved. So we have to do the wraparound ourselves.

Issue Links

• is related to: SIS-486 Some map projections produce wrong results when |Δλ| > 180°