Bug #471

project option #3 not working correctly

Added by Mark about 4 years ago. Updated about 4 years ago.

Status:FeedbackStart date:2013-12-29
Priority:NormalDue date:
Assignee:Mark% Done:


Target version:-
Affected version:5.1.x Platform:


Option #3 does not work correctly... The documentation (and my experience with past versions of GMT) indicate that when -Q is set, then the output should be in kilometers with a step size set by -G. The example below illustrates the problem. The euler pole is set to the north pole, and the center point sets the reference meridian to the prime meridian. The points are set to increment from 0 to 111 km (= 1 arc degree) along a small circle with colatitude of 45 degrees. Inspection of the results will show that last point is 1 degree longitude from the first, with is not equal to 111 km. The difference between the starting and final point should be cos 45 degree change in longitude. I seems that the calculation is marking the distance at the equator and not on the specified small circle.

gmt project -C0/0 -T0/90 -G1/45 -Q -L0/111

By the way, option #3 would be easier to use if it were possible to specify that the -C point marked the start of the small circle path. Right now, one has to calculate the colatitude of the point to set up the calculation.


#1 Updated by Paul about 4 years ago

  • Status changed from New to Feedback

Per documentation, the -Ginc/colat says:

     Optionally, append /<colat> for a small circle path through C and E (requires -C -E) [90].

Whereas your command has no -E. Of course, it should not quietly give the wrong answer. We can probably make changes to make this work as well.

#2 Updated by Mark about 4 years ago

It was my understanding that option 3 allowed one to use -L to specify, in which case -E is not needed. For my application, I am merely trying to generate points starting at the -C point and continuing over the interval indicated by -L along the small circle determined by -C and -T. I seem to remember being able to do this in previous versions of GMT....

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