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Datum level

 
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roddandtoddflanders



Joined: 02 Oct 2011
Posts: 24

PostPosted: Mar 09, 2012 12:01 am    Post subject: Datum level Reply with quote

Some people have noticed that our coordinates were off lately and we've been using averaging and it was suggested that it might be the datum level because they are out by about the same amount (30 m) each time.

ruylopez suggested that we try WGS84 instead of NAD83. However, it was on NAD27 and the only WGS we have as an option in our gps was WGS72. We had never really paid attention to the datum level before.

Does anyone know if this change will solve the problem? Or can anyone help us figure it out? He had suggested to try on here as well.

Thanks and sorry for the bad coords lately.
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ertyu



Joined: 25 Apr 2005
Posts: 552
Location: Winnipeg

PostPosted: Mar 09, 2012 1:59 am    Post subject: Reply with quote

What gps are you using that doesn't have WGS84?
That is 1984.

Using NAD27 would certainly add error to every coordinate if it is then interpreted as WGS84.
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roddandtoddflanders



Joined: 02 Oct 2011
Posts: 24

PostPosted: Mar 09, 2012 2:42 am    Post subject: Reply with quote

It`s a Magellan Explorist GC. http://www.magellangps.com/Products/eXploristseries/eXplorist-GC

We checked the menu a couple of times. Maybe we are missing something but it only shows one WGS.
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dani_carriere



Joined: 04 Feb 2005
Posts: 502
Location: Winnipeg

PostPosted: Mar 10, 2012 7:35 pm    Post subject: Reply with quote

If you cannot find WGS84 or a close alternative, you may want to mention that on your cache pages so people can change their units to NAD83.

I believe the Groundspeak forums may gave a Magellans user area (at least they used to). They may be of assistance.

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dani_carriere



Joined: 04 Feb 2005
Posts: 502
Location: Winnipeg

PostPosted: Mar 10, 2012 7:41 pm    Post subject: Reply with quote

Another alternative is to take your coords using NAD83 (or nad27) and using an online tool to transform it to WGS84 for posting to GC.com. Apparently, you're not the only ones to have this issue. Here is a link to a forum, and someone has listed some transformation sites that may be useful to you.

http://www.gps-forums.net/conversion-between-nad83-csrs-wgs-84-a-t36760.html

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ertyu



Joined: 25 Apr 2005
Posts: 552
Location: Winnipeg

PostPosted: Mar 10, 2012 7:45 pm    Post subject: Reply with quote

geocaching.com requires the coordinates in WGS84, don't post something else. If you must and as a last resort, convert them from whatever datum in use to WGS84.
However, that gps must support WGS84, the manual indicates it does, if your unit doesn't, it is broken.
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roddandtoddflanders



Joined: 02 Oct 2011
Posts: 24

PostPosted: Mar 11, 2012 12:30 am    Post subject: Reply with quote

Thanks for the help. We will look into this stuff more. Our gps has had some issues in the past too.
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dani_carriere



Joined: 04 Feb 2005
Posts: 502
Location: Winnipeg

PostPosted: Mar 11, 2012 3:16 am    Post subject: Reply with quote

If you haven't figured it out by the next event, let me know. I can take a look at it. Also I'm sure there are other Magellan users out there who may know what to do.
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IAMSTIG



Joined: 12 Aug 2009
Posts: 31
Location: Winnipeg

PostPosted: Mar 13, 2012 7:36 pm    Post subject: Reply with quote

You shouldn't use online tools to do transform datums. It is not a conversion that can be done mathematically (even if the program claims it can). A datum is really the shape of the Earth on which the projection is over laid and it has been updated over the years as more accurate information has become available. You can easily change from one projection to another using math as the underlying shape of the Earth remains the same but if you change the Earth's shape then there is no single common reference point to do the conversion. The only way to do it properly is to use a lookup table from a known grid and use those points to look up the offset at that location. You can download the grid from the ESRI site here if you really want to... http://www.esri.ca/en_resources/1470.asp
Quote:

The National Transformation Version 2 (NTv2) grid shift files provide a national standard for transforming coordinates between the NAD27 and NAD83 reference systems. To use the Canadian National Transformation Version 2 method in ArcGIS you will need to download the appropriate files.


If you can't use WGS84 then use NAD83...they're basically the same thing for the accuracy we're talking about here. NAD27 will be off about 30m one way and 60m the other...plus or minus depending on where you are etc.

Basically consider NAD83 and WGS84 to be the same thing and enter the coords on the geocache site as they come off your GPS assuming you collect them in either of those formats. If you use NAD27 you can not easily convert them to NAD83 or WGS84 without the use of proper mapping software that uses the lookup tables.
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