Datums

[petnic]

Hi
Im altering some datums from AUS 66 to WGS 84 and im not sure how the last digits go for example 33.34.17 in AUS 66 .....are the last two digits 017 or 170? in WGS 84

cheers
Pete

[Fishing_Freddy]

Hi pete I think the zero goes on the end not a hundred percent sure but you could try it then see if you end u in hte same place
Ken

[Kerry]

Hi
Im altering some datums from AUS 66 to WGS 84 and im not sure how the last digits go for example 33.34.17 in AUS 66 .....are the last two digits 017 or 170? in WGS 84

cheers
Pete

....AUS 66 to WGS 84 and im not sure how the last digits go for example 33.34.17 in AUS 66 .....are the last two digits 017 or 170? in WGS 84

Pete, Need a better explanation as if it is 17 in AUS66 it won't be 17 in WGS84 but in any case in any coordinate/datum the zero or zero's are always implied on the end.

Writing it thw way you have written it 33.34.17 is the same as 33.34.170 is the same as 33.34.1700 etc

HOWEVER! 33.34.17 as you have written it means absolutely nothing as it could be 33° 34' 17" (DMS) or 33° 34.17' (DM.m) and you really can not covert from one datum to the next properly unless you know the intended position format to start with.

Regards, Kerry.

[petnic]

This is how the AUS66 datum appears Kerry 33° 43' 11" 151° 20' 53"

Thanks for all your help

cheers
Pete

[Kerry]

33° 43' 11" 151° 20' 53" (AUS66)

33° 43' 05.3" 151° 20' 57.1" (GDA94/WGS84)

Precision of the AUS66 coordinates (whole seconds) is no better than 31 metres, which is quite a bit more than what normal GPS accuracy is.

Precision of the above WGS84 coordinates (to 1 decimal second) would normally be around 3 metres BUT as the original coordinates are whole seconds then a comparable precision would 33° 43' 05" 151° 20' 57" (GDA94).

Regards, Kerry.

[Cheech]

Pete, your gps should have both datums in it. Just change your datum to AUS66 and set the units to degrees, minutes and seconds to match the format you have (00.00.00). Enter in the mark you have,,, then change your datum to wgs84, and change the units on your gps to degrees, minutes, and fractions of minutes (000.00.000). Your gps will automatically convert the mark from AGD to WGS and from degrees to fractions of minutes.

Like most of us, I am assuming you are expecting something like 00.00.0000 instead of 00.00.00 Once you have a mark correctly in your gps, you can change the units same as changing the datum, and the gps will correctly convert it for you.

But I am happy to be corrected. This is my understanding from datum conversions I have made from a map program I use that is AGD66 and converting them to WGS84. I Tested on the brisbane leads and it got me within a couple of metres.

Cheech

[Kerry]

Just be aware that not every GPS converts coordinates when the datum setting is changed.

The majority do convert as expected so that the physical point on the ground remains the same but with different coordinates relative to the datum.

Some however (some Lowrance models do this) do not convert the stored coordinates with the same coordinates in different datums then being different physical ground points.

Regards, Kerry.

[petnic]

Thanks for all this help Guys. Im struggling to get my head around this

I have a garmin 72 and what seem the main options in units are
hddd.dddd
hddd.mm.mmm
hddd.mm'ss.s"
I dont have the option of 000.00.00 which is what i seem to need

So do i choose AUS66 and hddd.mm'ss.s ??

input the waypoints

Change to wgs84 and alter the units to hddd.mm.mmm ?? and i should be right

cheers
Pete

[Kerry]

Thanks for all this help Guys. Im struggling to get my head around this

I have a garmin 72 and what seem the main options in units are
hddd.dddd
hddd.mm.mmm
hddd.mm'ss.s"
I dont have the option of 000.00.00 which is what i seem to need

So do i choose AUS66 and hddd.mm'ss.s ??

input the waypoints

Change to wgs84 and alter the units to hddd.mm.mmm ?? and i should be right

cheers
Pete

As long as you are certain of the source coordinate format (Deg Min Sec) & Datum (AUS66) then that's exactly what you do.

For a value of 33° 34' 17" simply input as 33° 34' 17.0"

Just remember any converted coordinates will only ever be as accurate (precise) as the the original coordinates to whole seconds and good to about 31 metres at best out to about 44 metres.

Also being only listed to whole seconds they would have probably been recorded pre 2000 in the days of Selective Availability when GPS units could not use and did not require precision better than whole seconds based on a system accuracy of around 100 metres. When looking for these marks you might have to widen the search radius until you locate the intended spot?

Regards, Kerry.

[petnic]

Kerry.
It all seems to have worked a treat, hopefully when i give it a run this week i'll be in the right area and somewhere close .
Many thanks again
Pete