Manitoba Geocaching Association

roddandtoddflanders · Joined: 02 Oct 2011 Posts: 14

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.

ertyu · Joined: 25 Apr 2005 Posts: 547 Location: Winnipeg

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.

roddandtoddflanders · Joined: 02 Oct 2011 Posts: 14

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.

dani_carriere · Joined: 04 Feb 2005 Posts: 481 Location: Winnipeg

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.

dani_carriere · Joined: 04 Feb 2005 Posts: 481 Location: Winnipeg

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

ertyu · Joined: 25 Apr 2005 Posts: 547 Location: Winnipeg

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.

roddandtoddflanders · Joined: 02 Oct 2011 Posts: 14

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

dani_carriere · Joined: 04 Feb 2005 Posts: 481 Location: Winnipeg

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.

IAMSTIG · Joined: 12 Aug 2009 Posts: 31 Location: Winnipeg

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