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Hi Folks,

 

I'm trying to work out what's going on here.

 

 

    I have a series of lines in WGS84. I run them through this process:

 

 

LengthCalculator -> Reprojector (to _AZMED_) -> LengthCalculator.

 

 

This gives me two lengths for each line, one in WGS84 "units" (meaningless themselves), the other in _AZMED_ units (metres I think).

 

 

I expect the absolute values of these lengths to be different, but the relative values to be the same (i.e. that I can sort by either length column, and that the order will remain unchanged).

 

 

However this isn't the case - the relative values are different.

 

 

Sorted by Azmed you see:

 

 

 

Sorted by WGS84, it's obvious that the azmed order is different.

 

 

 

 

However this doesn't seem to happen with polygons and the AZMEA projection - their sort order seems to remain consistent.

 

 

So the question - what quirk of projections am I missing? Which distance is "correct"? I know that different projections have different traits, but I didn't expect the WGS84/AZMED change to alter results so drastically for distances that are so similar.

 

 

Thoughts welcome.

 

 

Thanks,

 

Jonathan
Hi Jonathan,

 

 

The same distances in degrees along x axis (west-east) and y axis (north-south) are different in meters naturally. In higher latitude area, the difference is larger. e.g. 1 second (1/3600 degrees) is:

 

- in latitude 50 degrees, about 20 meters along x-axis; about 31 meters along y- axis.

 

- in latitude 60 degrees, about 15 meters along x-axis; about 31 meters along y-axis.

 

I think that it caused the order change.

 

 

Takashi

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