Geodetic Reference Systems, TRX, INDIR, Distance Calculated

[jonathan2]

Hello,

I'm a materials engineer so i don't have a background or any schooling in geomatics and haven't bothered to do much reading in the field. I've been messing around with some online applications provided by the government of canada.

http://www.nrcan.gc.ca/earth-sciences/geomatics/geodetic-reference-systems/tools-applications/10925

Particularly the TRX and INDIR application.

I set myself the following problem:

Compare distance calculations between 2 points using INDIR and TRX application tools

Point 1: N53d 32m 31.63s 113d 39m 9.06s ** W positive
Point 2: N53d 25m 59.66s 113d 35m 9.25s

The results I get ("hopefully i didn't make any stupid data entry errors")

INDIR: 12899.428
TRX: 12899.049 (converted lat/long --> UTM --> distance = sqrt( (x2-x1)^2 + (y2-y1)^2 )

I got a difference of 379 mm between the two applications and am not quite sure why... perhaps a geomatics engineer can shed light on the findings or my stupid entry error.

 

[jonathan2]

okay i've done some reading...

it appears TRX and INDIR are based on the same ellipsoid so thats a good start...

"The NAD83 is an earth-centered datum based on a newly defined ellipsoid – the Geodetic Reference System of 1980 (GRS80) – and its reference point is the center of the earth, as opposed to a point on the earth's surface."

I used GRS80 in INDIR and NAD83 in TRX...

I have noticed that my calculation of the distance is smaller than the one provided by INDIR and so i'm thinking that i don't quite understand what "distance" means between two lat/long points. I obviously calculated a cartesian based straight line distance between the two points which is probably simplistic. My hunch is that I need to calculate the arc length of the ellipsoid? so my question is much simpler now...

What would be the proper equation for calculating distance between 2 points in lat/long or UTM (preferable). I would like it to be based on the ellipsoid. A link to a good website would be sufficient.

 

[jonathan2]

http://geographiclib.sourceforge.net/

--> free library saves me the coding i would have done when provided an equation.

also after realizing my fault with the basic Cartesian distance formula. i requested output from TRX in Cartesian coordinates and recalculated the distance.

ans = 12899.425

3 mm difference now... after the reading i've done i believe they should be the same value unless there is some loss of accuracy by performing the transformation to cartesian coordinates??? perhaps series truncation errors?

why 3 mm difference? that's the new question...

i wonder if anyone is enjoying reading my thoughts and watching me answer my own questions... i guess if someone finds this thread useful i suppose its still helpful.

 

[IRstuff]

I don't know about your specific problem, but I've run across different distance formulas, and they can produce errors on the order of millimeters, which was OK for me, since I was concerned about total distances on the order of kilometers, so a few ppm weren't a big deal

TTFN
faq731-376

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[jonathan2]

this is for 3D pipeline modelling in FEA software packages for stress analysis. 3 mm is acceptable error... so i'm satisfied now.