Re: Diference in km between two GPS cordinates.

Brad White wrote:
WillR brought next idea :
Marcello Dias wrote:
Does anybody have the code to calculate the diference IN KM between two GPS cordinates like:

Nobody mentioned datum, although one poster mentioned ellipsoids. If you do not know the datum of the coordinates the information is not useless for a general ideas of your location -- but for for mapping work the answers you will get will not be sufficient -- i.e. useless.

Also -- your answer may depend on the "projection" under which the coordinates were taken...
http://erg.usgs.gov/isb/pubs/MapProjections/projections.html#miller

I would have thought that these didn't matter so long as
they are using the same datum and projection.

When I was working with this, we never received this info
with individual results. You had to know from the database.
And each datasource was expected ot be self-consistent.
So as long as your two data points are coming from the same
data source, I would think you are OK.
If they are different sources, then all bets are off, as
they will be different dataums etc, in a significant number
of cases.

Brad.


Bear with me..

First -- you cut this part...

-----------------
"Is this a "consumer" type thing -- location to a store -- or a scientific purpose -- like for exploration mapping -- or perhaps mapping for property and municipal zoning mapping?

What is the purpose of the calculation? Is this a student or learning thing? If so quit reading -- other posters gave you the answer."
-------------

So I already covered this -- if this is a "flaky calculation" as in you are 154km from the Gee-Whiz Superstore -- who cares? What he did is more than good enough and I acknowledged that.


Now the rationale behind the comments...

I believe he was posting from Brazil -- this is relevant.

You might want to look at Mexican Maps for a good (bad?) example of what I am saying. And I can clap together an example more quickly from their situation since the memories are more recent.

The system used by the Mexican mining recording office (DGM) is to reference to NAD 27 datum maps INEGI produces ITRF90(?) datum maps -- essentially WGS84... so if you use one set of coordinates to calculate a distance -- it will be the same distance as on the other set of maths -- according to the math -- not so. The NAD 27 contour map for Mexico routine shows errors of up to 1.5KM (A Mile) and in places is off by 6km. So it depends on how you got the coordinates. If you read them off the NAD27 map then -- they are a set of coordinates which are meaningful to a degree known only to -- uh gee I dunno. Now if you measure the coordinates on a GPS -- then apply them to the map -- very often you find yourself looking at a ridge when you clearly remember a valley when you took that set of coordinates -- so all the calculations in the world may not help you to get a reasonable calculation.

Now I do not know the state of the mapping science in Brazil (his coords were from that are of the world I believe) -- but in Nebraska you can probably get centimeters accuracy from your state plane system -- which has been fiddled until the maps are precise and accurate.

Another issue s that he was using a float -- not a double -- so if they were_ accurate coordinates in a reasonable datum -- the calculation _may_ be worthless if he needs to post a leg length in a mineral claim. Particularly if he is using Relative Polars to calculate lengths in a closed figure

So even if his calculation is correct -- if a user of the data does not choose the same datum to measure the same distance -- you should get a different answer -- it depends on the mapping technology of the part of the world in question. The wgs84 distance should (almost) always be consistent -- it too depends on _exactly_ where you are, the time of day, the phase of the moon etc. (no joke) if you are doing maps where extreme accuracy is required.

So -- since he never stated the reason for the calculation...

I stand by my claim that these calculations are "context sensitive" -- at least in many parts of the world.


Simplest way I can think of to say this is... The only really accurate data is the "raw data" from a GPS -- the Great Circle so to speak -- once the GPS applies a datum to the data it is making an approximation to a mapping system -- a DATUM which depends on an ellipsoid --> a map is based on a given projection, and a DATUM (Ellipsoid derived) -- this simply gives you an error estimate.

The ellipsoid is the closest match to the pear shape of the earth. And when you project all this confusion onto a flat piece of paper you get a little bit more confusion. :-)

Conclusion: Those who can work in a State Plane Coordinate System should be considered fortunate indeed!

Once burned -- twice shy!

I do not claim to be an expert -- I could happily hire one to do this work... lol



--
Will R
PMC Consulting
.

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