minutes, seconds, ft., in.,

[Guest bacpac]

quote:

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Originally posted by Cape Cod Cache:

OK, here we go.. Degrees, minutes, seconds have to do with geometry of a sphere (Earth).

Longitude (north-south lines) can't be directly converted into distance without some math and astronomy,

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UTM format uses distance to define coordinates. That is why it is so popular and useful on topo maps. I hope the USGS continues to use the UTM format.

[Guest cva38bw]

Not knowing much about map reading. Can someone explaine to me, when reading a topo zone map and I put in degrees, minutes, seconds. It shows it in degrees, ft., in.. Am I just reading it all wrong? I'm working with WGS-84

 

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[Guest BigFig]

W83 07' 44.33"

 

That's degrees (83), minutes (07) and seconds (44.33). Yeah, I know it's the same notation as feet and inches use. Also, geocachers usually give coordinates in degrees and decimal minutes: W83 07.739' - most GPS units will let you set up your coordinate notation to degrees and decimal minutes.

[Guest BigFig]

BTW - while geocachers by convention use WGS84, most topo maps use the NAD27 datum. So, if you are taking readings from your GPS and plotting on a topo - or vice versa, you will probably need to convert the coordinates from NAD27 to WGS84. You can do that right on your GPS by taking the reading or setting the waypoint using one datum, then switching the datum on the GPS. The GPS will automatically convert the coordinates. Depending on where you are located, the differences between NAD27 and WGS84 can be as much as 100 meters.

[Guest bacpac]

I reiterate one of my previous posts...

 

"In my opinion the significance of Map Datum is poorly defined in the literature of some GPS manufacturers.

 

I assumed that the default map datum of my Garmin GPS12 would be the same as all the USGS topos I owned. It is not. I have not seen any maps that use WGS-84 datum, but I assume there must be some.

 

I advise each GPS owner to study their maps and the section on map datum in their owners manual. GPS software has not advanced to the level of PC software that facilitates custom user interfaces."

[Guest Cape Cod Cache]

OK, here we go.. Degrees, minutes, seconds have to do with geometry of a sphere (Earth).

Longitude (north-south lines) can't be directly converted into distance without some math and astronomy, they ARE convertable to time. 360 divided by 24 hours = 15 degrees (time zones) Latitude lines (east-west) are distance. WGS 84 is the standard because GPS were primarialy used for marine and aviation navigation. both types of charts are made by NOAA (Dept of Commerce), while USGS (Dept of Interior) topo maps still use NAD 27. USGS maps and large scale NOAA charts also use Mercator Projection (make a sphere flat) just to help screw things up even more!!! NAD 27 is 74 years old and made for/from North America only, WGS 84 is 17 years old and done for the globe. Why the USGS hasn't updated is beyond me, but all the different gov't agencies are hoping to get their collective acts together over the next few years. I hope that helps a bit and doesn't give you nightmares of trig class.....

[Guest KK6T]

Not having a map in front of me......do all maps (topo, etc) list the format they are using?

[Guest bob_renner]

quote:

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Originally posted by KK6T:

Not having a map in front of me......do all maps (topo, etc) list the format they are using?

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All topo maps I've seen have the datum listed in the bottom left corner of the map. Most all are NAD27, but I think some of the newer ones are WGS84 or something new like that. They also have some text there telling how to translate from NAD27 to the newer datum (e.g. move 60 meters east and 3 meters south). This is also reflected in tick marks near the corners of the printed portion of the map.

 

The topo maps themselves are drawn using DMS as the edges of the map - every 7.5 minutes. Along the edges are more tick marks for both DMS and UTM. There may or may not be a grid of lines drawn on the map using one of these coordinate systems - generally UTM on the recent maps.

 

Other maps (like state highway maps or street maps) generally don't have any reference to datum or coordinates. Some of the more detailed atlases may have coordinates shown, but I haven't seen any listing what datum they used. Unless you're zoomed in close, as the 7.5 minute topo quads are, the difference in datums will be less than the resolution of the printing of the map.

 

Bob Renner

[Guest Cape Cod Cache]

Here is what I found... I have a bunch of marine charts. An old unit (1978) uses NAD 27, has Nantucket Lightship on it. USLESS !The NOAA (US Dept of Commerce) and USGS ( Dept of Interior) cant figure out what to use

Pretty sad.

United States Coast Guard (NOAA) charts use WGS 84. I'll use that format, quads dont happen over water.

[Guest fairbank]

Cape Cod Cache wrote:

United States Coast Guard (NOAA) charts use WGS 84. I'll use that format, quads dont happen over water.

 

Oh yes they do. I use USGS topo maps for all my kayaking trips in the rivers, creeks and streams of the Chesapeake Bay area. Many of the quads even have the depth contours on them, not that I care about depth in a kayak.

 

Eric

Topo map help page:

http://www.geocities.com/fairbank56

[Guest 300mag]

Personnaly i use wgs84 for geocaching but thats all.In any other circumstance i use nad27 since all of the maps in this area use this datum,I also use utm grid coordinates instead of the old degrees mins secs..I find these utm a lot more easier to use when using my maps.

 

[This message has been edited by 300mag (edited 18 April 2001).]