Coords errors

**WhiteD3** · 25th December 2007, 11:45 AM

Hiya,

Recently picked up a Garmin Legend Cx and used it the first time on the Boonah trip a fortnight ago.

I had it logging for most of the trip, downloaded the track and waypoints to Mapsource and saved it for another day. Djhampson posted his track data (boonah......16th dec 07) so I thought I'd do some comparisons.

Thing is, there's a big error between his track and mine. ie I've compared the same point (the base of Fiat Hill) but the two points are a long way apart.

Here's Djhampson's track on Google Earth with Fiat Hill shown.

Here's my Mapsource data showing the same spot as per my GPS. The point "Fiat2" is the same point above, I've just copied the coords.

and below showing the waypoint properties of "Fiat2" as copied from the Google info of Djhampson.

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So what have I done wrong?

tombraider · 25th December 2007, 12:02 PM

What Datum are you using in Mapsource?

What datum is your GPS set to?

WGS84 is the common one...

And dont trust googles co-ordinates

tombraider · 25th December 2007, 12:04 PM

Whoa!!!

I think I spotted it...

Google is in DMS.sssss

Mapsource is DM.mmmmm

**sschmez** · 25th December 2007, 04:06 PM

Originally Posted by tombraider

Whoa!!!

I think I spotted it...

Google is in DMS.sssss

Mapsource is DM.mmmmm

That looks to be part of it ....

also, it's the tip/point of the "Pin" that indicates the placemark in google earth. Zoom right in and then adjust your placemark. (right click the "pin", properties and then just drag it around to where it should be)

In Google earth go to Tools -> Options
and you get three choices:
DMS
Decimal degrees
UTM

use one of these options in mapsource as well and you'll be comparing apples to apples

**JDNSW** · 25th December 2007, 04:53 PM

NO map coordinates are 100% trustworthy, even if you have understood and taken into account all differences in datum and projection.

Just as an example, the "whereis" coordinates for the village near where I live are in error by over 20km, and consequently the directions it gives are not only unreliable but downright dangerous - the location it leads you to is on a very poor forest road, with nothing there, and no phone coverage.

But even "accurate" coordinates have errors, often due to differences in projection and datum, but sometimes simple inaccuracy. One of the problems with any compilation map (paper or digital) is that in general you have no idea where the data came from or how accurate it is.

John

**WhiteD3** · 27th December 2007, 03:56 PM

Thanks Guys.

The problem seems to be that the datum in Mapsource was set incorrectly. Now changed to WGS84 and the units are now set the same.

There's still a small error but it's looks to be down to 10m or so.

Thanks for the

education!

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