datum conversion

[+frefel]

I am looking for a cache in which the coords are given in the normal WSG 84 datum which leads to a bogus site. To find the real cache site the datum has to be converted to Old Hawaiian datum and searched with it. I have the Old Hawaiian datum as a choice on my eTrex Vista C but changing to it does not alter the obviously incorrect location.

How do I use the datum selections in the GPSr to convert from one to another?

Thanks in advance.

[+Unobtainium]

The physical numbers will not change, but the location shown on the map should as should the location on the ground. The difference is likely to be only a few hundred meters at best, so you may not see it o n the map, depends on the detail of the map you have.

[+dfx]

I would think that you need to switch to the other datum first, then create the waypoint at the given location and then change the datum back to WGS84. Looking at the waypoint's coordinates should then give you the converted numbers. Disclaimer: untested.

[+Team Dougherty]

I would think that you need to switch to the other datum first, then create the waypoint at the given location and then change the datum back to WGS84. Looking at the waypoint's coordinates should then give you the converted numbers. Disclaimer: untested.

 

That is how I would do it.

[+StarBrand]

I would think that you need to switch to the other datum first, then create the waypoint at the given location and then change the datum back to WGS84. Looking at the waypoint's coordinates should then give you the converted numbers. Disclaimer: untested.

 

That is how I would do it.

me too.

[+fizzymagic]

I would think that you need to switch to the other datum first, then create the waypoint at the given location and then change the datum back to WGS84. Looking at the waypoint's coordinates should then give you the converted numbers. Disclaimer: untested.

 

That is how I would do it.

me too.

 

Not me. I wrote my own datum converter for that cache. Used the 3-parameter Molodensky transform. Worked great. I don't know how many GPS units have the Old Hawaiian datum in them.

[xyzee]

All of my 60 series have the Old Hawaiian Data in them, as well as the SporTrak and Meridian.

 

A quick test shows that there is about 489 yards difference between WGS 84 and the O Hw. That's a bunch. Think I better run that again!

[+frefel]

Thanks everybody - we found it today.

 

What worked was to enter the given coords into the Old Hawaiian datum setup on the eTrex then change to the usual WSG 84 format. The result was a different set of coords that led us to the prize. All just like dfx and others suggested. And yes the distance from the WSG 84 bogus site to the Old Hawaiian find was certainly hundreds of meters.

 

I had tried a NADCON (NAD 27 to NAD 83) conversion app but the results were quite a bit off. I assume that is because the Old Hawaiian datum is not exactly the same as NAD 27 even though it was stated to be derived from it.

Edited May 11, 2011 by frefel

[snowfleurys]

The Old Hawaiian datum is not the same as NAD27, It is specific to Hawaii, NAD is specific to North America.