Why does my Garmin 12 show the Altitude as 12m when I'm standing in the water at low tide?
Using 1:150000 maps ( eg AUS827/AUS828) I can't plot WGS84 readings to and from these maps - and they are diabolically bad when used with Oziexplorer. Any tips?
What is the best format to use on the GPS a) hddd mm ss.s or b) hddd mm.mmm?
It can depend on how many satellites it sees and which ones it is using for your location!Originally Posted by whiteman
What MapDatum do the maps use?
You must use the same Datum if your referencing off a map or copying points from another source!
Once again it depends on which format you're getting your points and references from!
All should be explained in the User Manual!
Rev.
Rev!
"Go n-ithe an cat thu is go n-ithe an diabhal an cat"
Don't take all that much notice of the height/altitude as displayed by many recreational receivers, it's not all that accurate and generally about 2-3 times less accurate than the horizontal.
And diabolically bad they should be too if trying to work in WGS84, as the datum of these charts aren't WGS84.
AUS827now that should be a 1964 version! Horizontal datum, Unknown. AUS828 (last edition 1990)at least has a datum, AGD66, which is somewhat different to WGS84.
Stick with hddd mm.mmm for this type of application.
Cheers, Kerry.
Thanks guys for the info ....
Kerry, I actually went searching the web for articles I'd seen referenced by you in the past. Couldn't find a lot (do you have a web site?) only what you'd posted at Fishnet.
When I lived in Sydney I had accurate maps coming out of my ears - but no fish - as I worked for the software company that built the GIS for the NSW Land Information Centre. Up Townsville/Hinchenbrook way, we have the fish but the maps are at a very high scale and the main area I use is on AUS827 and this map sucks!
We have to carry charts on our boats but at this scale, no wonder tankers are running into the reef!
Whiteman, the charts are available in electronic format BUT then one requires the third party licenced software to drive them.
SeaFarer RNC can be obtianed on CD and the SeaFarer ENC charts are partly complete and for around your area are due Mid 2003.
The good old paper chart appears to be playing catch up with the change to the new datum and the requirement for IMO compliant systems.
Up until a few years back (about 1985) we had one chart for this area that was last surveyed from the HMAS Fly in 1854, however since been updated (I think).
There's been a lot of Aircraft Laser Depth soundings done through the reef channel areas for the ENC (and RNC) charts, and I reckon there's a few other reasons why rocks jump up in front of ships
All those articles I do have available on line but only individually, don't have an index set up (yet). One of those jobs to do, One day.
http://www.cqnet.com.au/~user/aitken/gps/datum.htm
http://www.cqnet.com.au/~user/aitken/gps/format.htm
There's some others (I think, when I find them).
Cheers, Kerry.
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