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What Datum?


Guest peejay

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Guest peejay

What datum does everyone use? I use the Geo Datum 49 and would like to find a conversion routine to convert to and from WGS-84 so I dont have to switch back and forth on the GPS unit when entering data. Anyone know of such a beast?

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Guest rediguana

Hi Peejay.

 

I use the WGS84 datum as I believe it is the closest thing we have to an international standard - except for the distortion that occurs near the poles, but not many people go there icon_smile.gif Also I assume that most people posting caches are recording their waypoints using WGS84.

 

A quick search brought up a suite of utilities by the National Imaging and Mapping Agency, of the USA. If you register you can download this suite which contains a program called dtcc4 which converts datums.

 

NIMA MUSE Homepage

http://164.214.2.59/geospatial/SW_TOOLS/NIMAMUSE/

 

You may be able to find more details on my Google search for other programs.

 

Google Search

http://www.google.com/search?q=convert+new+zealand+geodetic+datum+1949+to+wgs+84

 

Cheers Gavin

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Guest rediguana

Hi Peejay.

 

I use the WGS84 datum as I believe it is the closest thing we have to an international standard - except for the distortion that occurs near the poles, but not many people go there icon_smile.gif Also I assume that most people posting caches are recording their waypoints using WGS84.

 

A quick search brought up a suite of utilities by the National Imaging and Mapping Agency, of the USA. If you register you can download this suite which contains a program called dtcc4 which converts datums.

 

NIMA MUSE Homepage

http://164.214.2.59/geospatial/SW_TOOLS/NIMAMUSE/

 

You may be able to find more details on my Google search for other programs.

 

Google Search

http://www.google.com/search?q=convert+new+zealand+geodetic+datum+1949+to+wgs+84

 

Cheers Gavin

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Guest peejay

Thanks for the lnk, I'll have a look. I thought that WGS84 was an American datum and did not correlate to our maps here? I use topo maps and find the NZ '49 datum works spot on whereas the WGS84 is quite a bit out.

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Guest rediguana

quote:
Thanks for the lnk, I'll have a look. I thought that WGS84 was an American datum and did not correlate to our maps here? I use topo maps and find the NZ '49 datum works spot on whereas the WGS84 is quite a bit out.

 

WGS84 was defined by the American Department of Defense and the National Imaging and Mapping Agency, but it is a global standard. There are important distinctions between local geodetic datums and global gedetic datums. For something like geocaching, with its global focus - WGS84 makes a lot of sense. As the GPS system uses a global geodetic datum, which I believe is WGS84, it makes sense to use the GPS units in that configuration most of the time.

 

Our only problem occurs when the WGS84 datum doesn't match with the NZGD49 - I believe there are differences of around 200-300m.

 

Most geodetic datums are tuned to local specifics of the earths crust - that is they are tweaked to a local best fit. However these systems don't scale to provide a good coordinate system for the whole world, which is essential for mapping things like GPS satelitte coordinates, and aircraft navigation.

 

There are a couple of good descriptions here: -

 

The European Air Traffic Control Harmonisation and Integration Programme

http://www.wgs84.com/wgs84/wgs84.htm

 

US National Geodetic Survey FAQ

http://www.ngs.noaa.gov/faq.shtml#WGS84

 

NZGD49 does provide the best mapping to the topo maps, but I believe that NZGD2000 will be more closely aligned to the WGS84 datum when it becomes widely used. I believe that one of the main reasons for the development of the NZGD2000 is so that NZ's reference frame is more closely aligned to that of the WGS84.

 

I use the 260 series topo maps, and WGS84 is a pain for them. My solution has been to enter all the co-ords in WGS84, and then switch to NZGD49 datum and NZMG coords, record the NZMG coords, switch the GPS unit back to WGS84 and d mm.mmm. The GPS transform functions seems pretty accurate and I haven't had a problem yet with this transfer. I do it this way as most of the coords I use are wgs84, and the only time I need NZGD49/NZMG are for the topo maps.

 

Hope that sheds a little insight into the various datums we have to deal with!

 

Cheers Gavin

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Guest rediguana

Haven't looked for a specific date, I believe it is a gradual thing. This page should have more information.

 

LINZ page on NZGD2000

http://www.linz.govt.nz/services/surveysystem/osgpublications/nzgd2000_index.html

 

Update! icon_smile.gif I just found this page which has some very good information about NZGD2000.

 

New Zealand Geodetic Datum 2000

http://www.linz.govt.nz/services/surveysystem/osgpublications/fsgd2000.htm

 

Cheers Gavin

 

[This message has been edited by rediguana (edited 02 June 2001).]

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Guest rediguana

Haven't looked for a specific date, I believe it is a gradual thing. This page should have more information.

 

LINZ page on NZGD2000

http://www.linz.govt.nz/services/surveysystem/osgpublications/nzgd2000_index.html

 

Update! icon_smile.gif I just found this page which has some very good information about NZGD2000.

 

New Zealand Geodetic Datum 2000

http://www.linz.govt.nz/services/surveysystem/osgpublications/fsgd2000.htm

 

Cheers Gavin

 

[This message has been edited by rediguana (edited 02 June 2001).]

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