Map Datums

[Guest bacpac]

Why doesn't this site use UTM coordinates as a standard? UTM coordinates are much more intuitive and is the datum used on USGS maps.

Why use degrees, minutes, and seconds when coordinates can be logged in meters? If people are not familiar with the UTM system it would be well worth the short time to learn it.

 

For exanple: If I am standing at 15S 0369238/3923896 and my objective is 15S 0369238/3923696 then I know that my objective is 200 meters south of my current position just by subtracting the coodinates.

[Guest bob_renner]

If you're standing at 11S 773076/3900000

and your objective is 12S 226901/3900000

how far away and in what direction is

your objective?

 

First of all, UTM is NOT a datum. It is a coordinate system. Both UTM and DMS use WGS84 or NAD27 or some other datum. Also, USGS topo maps are all made with the borders using DMS with the spacing being 7.5 minutes (for the newer 1:24000 series) and the NAD27 datum. They all have both DMS and UTM tick marks on the borders. Some of the newer ones are cluttered up with a grid printed on the map. Granted, the UTM system looks nice with its square grid of uniform spacing, but this nice look disappears at the border of two zones where the lines come together at angles.

 

Since most people who use this site have a GPS receiver, once they enter the destination (be it UTM or DMS) the receiver will tell them what direction and distance it is to their destination. At that point it doesn't matter what coordinate system they use. It's all a matter of personal preference, and I see no reason to change the main display now. You can always convert the coordinates either on the web page with the conversion feature or in your gps after it's entered.

 

Bob

[Guest Alex]

UTM grid is fine. But when, as you seem to expect, it is based on NAD27 datum it is fairly useless to me. Just as AUS84 datum would be useless to you and OSGB is useless to both of us.

Lat/Long using WGS84 is the GPS standard and so should be the geocaching standard. It is not a big problem to program your GPS in WGS84 Lat/Long and then make it convert to your preferred system - and if you are lucky enough to own a Magellan 315 you can run two systems at once!

[Guest bacpac]

What is the distance and direction between N35 14.081/W94 19.593 AND N30 54.774/W089 27.382? I don't know, but perhaps there is a simple way of determining this that I am not aware of.

 

I normally plot several waypoints from my map before I enter an area. To determine my position I can look at my current position and the coordinates of any of the waypoints. This provides a simple and meaningful picture of my surroundings. I suppose this is problematic when near borders of the UTM grid, but I have not experienced this problem.

 

I would encourage everyone to give UTM coordinates a try.

[Guest bob_renner]

I have tried UTM and my personal preference is for degrees, either decimal or DMS. You made a statement that you could tell the distance and heading from one UTM point to another by just subtracting. That only works in a very few cases, and totally falls apart when you have to cross zones. The UTM system looks nice within a zone, but it is a mess when you cross zones. The eastings totally change and the angle of the lines changes. At least with DMS, everything is continuous even if the lines of longitude converge at higher latitudes. It's all a matter of one's personal preference.

 

Bob

[Guest bacpac]

I believe the subtraction method with UTM coordinates works in all cases EXCEPT where you cross zones.

 

I am still a neophite, but this seems much simpler than converting degrees to distance.

[Guest bacpac]

In preparation for my first GeoCache search I set the Map Datum to WGS-84 and coordinate format to Hddd.mm.mm before I loaded the coordinates into my GPS (Garmin 12).

 

Since my USGS DRG map is NAD 27, I changed the Datum back to NAD 27 and my coordinate format to UTM. Sort of a hassle and I screwed it up the first time, but now the datum and coordinates of my map and gps match. I assume the Garmin software is capable of handling this type of transition and my coordinates are correct.

 

Where would I get a map with WGS-84 datum to avoid this hassle? Could the coordinates be posted with multiple (2) map datums?

[Guest fairbank]

It's really a simple matter to change the datum of your map by adjusting the position of the UTM grid lines. I normally just stick with NAD27 that most of the USGS DRG's use, but tonight I was creating a map for a friend, and the two maps I was joining together were using different datums. This is the first USGS DRG that I have found that uses the NAD83 datum (virtually identical to WGS84). This quad was also created using aerial photo's from 1987/88 and the map date is 1991 so not all these maps are as old as some like to think. Anyhow, I wanted the grid lines to line up between quads, so I adjusted the grid lines of the NAD27 map to be in NAD83 so everything lined up.

 

Eric,

Topo map help page:

http://www.geocities.com/fairbank56