View Full Version : Datum Conversions

01-10-2001, 07:12 PM
I have acquired some GPS marks but are in datum AUS66 is there a program that can convert those marks to Datum WGS84 or do you know of an easy way how to do it??

02-10-2001, 01:20 PM
Many ways to do this.

Probably the easiest way (if your GPS allows it? and most do) is to change your GPS datum setting to the datum of the marks, AUS66/AGD66 or whatever the GPS supports. Input the marks as the 66 values then when you change back to your original datum (WGS84 I hope) the conversion should be in the selected datum.

If that didn't appear to work then we can do one of several things? From a general Lat/Long of your area we could compute an adjustment to convert AUS66 to whatever or mail em and I'll convert them direct.

Cheers, Kerry.

02-10-2001, 03:17 PM
??? ???the following conversions should work
latitude decrease by 5.51 (170m)
longitude increase by 3.97 (116m)
hopes this helps

02-10-2001, 03:21 PM
:P sorry slip of the finger should read
longitude increase by 3.96(116M)

02-10-2001, 03:28 PM
??? ???rushing into thing here
if you want so gps marks in wgs
i have every beacon from caloundra to nerang bridge
plotted, plus many waypoint offshore from top of morten to
the tweed, but they are only my waypoints i have plotted
you may get many for the one reef, if u need some give us a

03-10-2001, 04:38 AM
Thanks webby for the info unfortunetly I have been given a set of figures similar to yours but still not the same to add and subtract and as you can imagine now I'm at a bit off a loss cause who's numbers do you use!! ???
Thanks again for the offer of GPS marks but I live in sydney.
Kerry I will take up your offer and email the marks to you they are all AUS66 I just have to sift through them cause there are about 50 of them but I only need about 10 to 15. I should have it all sorted out in 2 to 3 days and thankyou very much.

03-10-2001, 06:39 AM
I don’t normally quibble about a few metres but since Webby has corrected the longitude from 3.97” to 3.96” it sorts of sounds precise, but I wouldn’t like to see myth turn into fact on that account. Corrections based on one area technically don't/won't be the same in another area (something like 30-40 metres difference across Australia)

Ok, a few questions for Webby? (main reason that any errors don't snowball on and on and become fact as they have appeared to be already?)

- Where did those corrections come from?
- Why the corrections in seconds?
- Why 2 decimal places?

Most recreational GPS’s don’t support that precision and besides Degrees Minutes Seconds is not (and I don’t believe it should be either?) a commonly used position format in a marine environment.

Based on lats/longs around southeast Qld (assuming those figures are based on that location) those figures don’t really match actual datum differences (between AUS66 & WGS84). Even based on those figures (and I don’t agree with the lat/long difference anyway) I’ll go along with the latitude metres v seconds straight conversion but not the longitude. There’s not a lot in it but enough to be about twice the average capable accuracy of the GPS system these days.

Like I said it’s not a lot when one is floating around the water but being out by twice the systems capabilities before one even begins might be a backward step.

Xfactr, no problem, 15, 50 or 500 won't make any difference. I'll look for the file in a few days.

Cheers, Kerry.

03-10-2001, 03:35 PM
:-X :-Xhave checked it out and the following applies
i was out with my distances.. but this will get you back on
agd(aus66) to wgs the difference is approx 217m
the actual location will be 105m out e/w and 190m n/s
subtract 5.6' from the given latitude and add
4.2' to the given longitude.
you may be able to short cut this (its works on my magellan
by doing the following)
... change datum on your gps to aus66/84/agd84
... create a waypoint and enter the gps waypoint you were
...change datum back to what you normally use wgs84
... the waypoint should now have change to the correct
reading for use with your gps
(check this it should be 5.6' less for latitude and 4.2' larger
for longitude)
if still confused go to website
and have a read, there are a few other sites, but
this one should help..

03-10-2001, 06:26 PM
Webby, a couple of corrections first, 5.6’ is written as 5.6 minutes (that little’ denotes minutes) which is about 10km (give or take a bit). Similar with 4.2’ (4.2 minutes) roughly about 8km. What they should be (even back on that referenced site is 5.6” & 4.2” (that little” denotes seconds). Why seconds for a marine environment? Maybe something else to become confused with, especially when incorrectly denoted.

Correcting that little problem with minutes that are meant to be seconds I’d go along with those values as being “close” enough (+/- a few metres) AS LONG AS they were being applied around Sydney latitudes (give or take a bit).

Apply those values (as above) for conversions around say Perth, Darwin, Hobart, Adelaide, Cairns and even Brisbane then one may/will be up the proverbial creek. The conversion from AGD (AUS66) to WGS84 is NOT CONSTANT but varies. Not a great difference in overall resultant shift distance but can certainly in individual lat/Long corrections.

Cheers, Kerry.