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breilly

WGS84 Basic Instruction

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I am developing plans to place my first cache and I do not understand the use of WGS84 datum format. When I place my cache, I will determine the longitude and latitude coordinates (that is all I have used to date when searching for caches). Do I need to translate these coordinates to the WGS84 datum format or is that done automatically by the web-site. I don't want to put a cache on the web-site without adequate information and I don't understand the WGS84 format. Any information would be greatly appreciated.

breilly in St. Louis

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Without going to far into explaining it ... you really don't need to translate anything ... set your GPSr to the WGS 84 Datum and to the decimal degree/minute format ... when you place your cache ... you will have the coordinates in the form required to load into the site. No advanced math or manipulations required. Couldn't be much more simple. As for a detailed discussion as to why ... well for the best summary you can find read the writings of "Markwell" sage and all round nice guy!

 

Teachings of Markwell .. Volume 1

 

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[This message was edited by Hawk-eye on March 28, 2002 at 07:19 AM.]

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To explain a little better, there are two (sometimes three) things at work in determining your position on the earth.

 

The first is the datum used. Think of a datum as a set of rules used to determine the "shape" of the earth and the starting point and orientation of the reference system. It’s necessary because the earth is not perfectly round. Because different datums interpret the shape differently, there are slight differences in positioning between the different datums. WGS-84 is the datum we use on geocaching.com and is generally the default datum for most GPSr’s. The other commonly used datums in North America are NAD-27 and NAD-83. There are many, many more, but these are the ones in common use for map and GPS work

 

The other part needed is the coordinate system. This is a way of determining where a particular spot is in relation to any other spot. Here, we use Latitude and Longitude. Another very common coordinate system used in mapping and GPS work is UTM. There are others here as well, but you need not worry about things like ECEF, MGRS, etc.

 

This final thing needed is something called units. This is really a subset of the particular coordinate system used. Some systems like UTM always use the same units, so this is not an issue with them. For Lat & Lon, it is an issue because there are many different ways to represent Lat & Lon. The one we use here is called Decimal Minutes. It is often represented like this: HDDD° MM.MMM. Other common unit values are DMS, (Degrees, Minutes, Seconds), and Decimal Degrees.

 

The bottom line is that you don’t need to worry about either of these once your GPSr is set properly to WGS-84 and HDDD° MM.MMM. (It likely already is if you’ve been finding caches.) I can’t tell you how to verify this on your GPSr since they are all different. Check your manual and look for Datum and Coordinate System, (some manufacturers call this "Position Format").

 

Best of luck.

 

Scott / Brokenwing

http://www.cordianet.com/geocaching

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