Waypoints and conversions between datums

[Morlers]

Can anyone help me with a simple formula to convert waypoints between AGD84 <> WGS84 for entry into my GPS when waypoint obtained from others. I understand most of the official maritime charts have been drawn in AGD66 datum to date and have a stamp on them noting the conversion formula from WGS94 to AGD66. I also understand these maritime charts will in future be progressively drawn in GDA94 (which is almost identical with WGS84) to standardise with world accepted datum of WGS84. Thanks.

Morlers

[bigbrian47]

morlers this is one for kerry cheers brian ;) ;)

[opimax]

giday morlers to convert from agd84 to wgs84 decrease the latitude by 0.095 of a minute & increase the longitude by 0.058 a minute in south qneensland waters. hope this helps. lee

[Kerry]

Morlers, Probably easier and reduces the chance of inducing errors to make use of the GPS and enter the coordinates direct in AGD.

This is of course unless you have one of the (older) rare makes/models that won't allow this but if this is the case then it will/should be obvious by inspection and then you'll have to do it manually for sure.

If you've got AGD84 or AGD66 (same thing anyway as far as conversions go) coords then change the GPS datum to AGD84 or AGD66, enter the AGD coordinates, change to WGS84 (GDA94 same thing) and the WGS84 coordinates should be different by "about" those differences noted by optimax.

The difference isn't a constant and varies from place to place but if you end up with similar (close) differences then it's done.

If it's not then if you have your rough location we can derive specific corrections to convert AGD <> WGS84 for your specific location.

Also conversion of marine charts (paper versions) to WGS84/GDA94 has been in progress for almost 10 years and there's more WGS/GDA charts now than AGD66. Marine charts in digital format are all WGS84.

Depending on how old your chart is there could already be a revision in WGS84. This is easily checked if you have a chart number.

Cheers, Kerry.
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[Morlers]

Thanks Optimax and Kerry for your help.

I have a Garmin 126 GPS (purchased 6/02) but wasn't aware that the unit would convert co-ordinates when datum changed. That's great news. Will make it very simple to enter other fishos waypoints.

The chart I have is #AUS 817 for Hervey Bay and was printed on 9/5/02. It has a stamp on the corners that says WGS84 cannot be plotted directly onto chart and it has a notation to the effect that co-ordinates must be moved 0.09 mins southward and 0.02 mins eastward. Maybe next edition of this chart will be in new format of GDA94.

I understand a bit more now and certainly appreciate your help. Thanks heaps.

Morlers :)

[Kerry]

AUS 817 appears to be of 1967 vintage with 9/5/02 being when re-printed. So it certainly is an oldy.

"0.09 mins southward and 0.02 mins eastward" be a little wary of these especially when dealing with south (-negative) latitudes.

Also that 0.02' simply don't add up, I'd check that one.

Cheers, Kerry.

[Morlers]

Thanks for the info Kerry. I certainly appreciate your help.

Morlers

[Kerry]

Morlers, been giving all this some more thought as that 0.02' was troubling.

I would suspect that Aus817 is not AGD66 at all and of an unknown horizontal datum. Both the published date and edition date is Jan 13, 1967 and with AGD66 only being proclaimed on October 6, 1966 one would suspect this chart is not AGD66.

What it is would be is anybody's guess as prior to 1966 there were about 20 different datums used in Australia.

With that I'd certainly be a little cautious with entering coordinates from this chart into a GPS as assumed AGD66 unless it is clearly shown on the chart. Use the lat/long shift note on the chart for determining WGS84.

Actually looking around the Hervey bay area there is still many old admiratly charts. Aus241, which is the Northern portion of Great Sandy Straights was published on Jun 10, 1977 and the datum is "other" even with a "critical area" clasification.


Cheers, Kerry.

[Morlers]

Thanks for the extra input Kerry. I would have thought that Hervey Bay (incorporating Inskip Point to Bundaberg) was a reasonably populous area (over 10.000 boat registrations) and that any charts put out by the Hydrographic Service would have been up to the minute but as usual we are only country Queensland, not the big smoke.

I wont be entering any co-ordinates from the chart but mainly from other boaties for good fishing sites they have found and passed on.

I must say that the research into datums etc I started has been eye opening to say the least and I have learnt a great deal. Your extra input is very much appreciated. Thanks. :)

Now all I have to do is locate some of these fish and catch them (easier said than done for me).

Morlers

[Kerry]

Hydro office would be concentrating on main shipping routes, port charts etc and if Hervey Bay charts are that old with things unknown then it might be one of those areas that require doing from scratch?

If anybody would know what charts are what around that area would be Sue Brown (HBMTC) as I'm sure see had something to do with (or know about) the Qld Transport Department chart revisions.

Some info on DATUMS (http://www.cqnet.com.au/~user/aitken/gps/datum.htm) and POSITION FORMATS (http://www.cqnet.com.au/~user/aitken/gps/format.htm) if your interested.

Cheers, Kerry.

[Morlers]

Thanks Kerry.

I checked them out, very interesting.

Morlers