# GPSDump 4.3.4. Distance Calculations

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This topic contains 7 replies, has 3 voices, and was last updated by  nxcl 9 years, 8 months ago.

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• #78

nxcl
Participant

<p>Can anyone advise on the method/algorithm(s) used by GPSDump 4.3.4 to calculate distances in its flight optimisations from .IGC data and output to its .kml file?</p>
<p>In particular, how it derives distance for ‘Out-and-return distance’, ‘Flat tiangle’ and ‘FAI triangle’. Is it using 400m radius Cylinders, or other similar mechanism, in determining ‘Closed’ flight distances? If so, EXACTLY, how is final distance determined?</p>
<p>Any info appreciated.
</p>

#372

SteinS
Participant

I have used the rules at http://www.pgcomps.org.uk/. Note that for the FAI and flat triangle the direct distance between TP1 and TP3 is used, not the sum of the dotted lines. GpsDump does not calculate any scoring, just the distance. For the loop flights the gap distance is listed at the end of position C for the out-and-return and position E for the triangles in the statistics section.

#373

nxcl
Participant

Stein, Many thanks. I think that clarifies and confirms what I needed to know

#379

nxcl
Participant

Sorry, but another question on GPSDumps distance calculations.

Could you clarify how GPSDump calculates distance between waypoints it reports in .kml output?

Is it correct to assume GPSDump reports Great Circle Distances between waypoints using FAI Earth Radius of 6371kms?

I use GSPTools to derive such GC distances, using the FAI’s radius of 6371kms but seem UNABLE to arrive at the exact same distances as GPSDump

As an example between the following waypoints….

Lat 51.742433 Lon -3.250483
Lat 51.574400 Lon -2.229950

GPSDump reports a distance of 73.05kms
Where as GPSTools reports GC Distance of 72.8327 kms

I’m unsure which is actually the correct answer, I suppose GPSDump is more likely to be correct, but I’m not sure why?

Any thoughts you may have to shed light on this discepancy would be greatly appreciated

Regards

#380

SteinS
Participant

I use Redfearn’s formulae with WGS84 values (semi major axis 6378.137kms, inverse flattening 298.257223563).

#381

nxcl
Participant

OK Stein, Thanks for that, I’ll check it out

#386

ste
Member

In FS Haversine is used with FAI’s radius.

In FsFlight under Tools > Calc. dist it can be tried.

For the above mentioned coordinates FsFlight get same dist as nxcl.

Note that the various instrument makers (GPS or flight instruments) does not all use the same methods so what a pilot see in the display will more often than not be different from what FS or GpsDump reports.

FS use a default errormargin of 0.5% (can be changed under Tools > Options) which should cover for any differences.

Stein-Tore

#387

nxcl
Participant

OK Stein, thanks for the extra info.

Regards
TS

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