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GPS coordinates question

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Offline stingrey

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GPS coordinates question
« on: August 11, 2007, 09:26:38 AM »
Moderator note:  split off from another topic.

I'm not understanding how these coordinates work... I've got a portable GPS (I got the new Rino 530HCx if you are keeping tabs on my other post where I was debating on which one to get) and I've entered some coordinates for other points, but the below points are different somehow... The points I entered for other locations are more like 29.234.135, 103.356.135 or something... The long string of #s doesn't seem to work properly...

I'm sure the info works and I just haven't spent enough time w/my new toy, but does anyone have a quick clue for me? I have my final to go on Monday morning and I'm scrambling to recover from the latest bombed exam (despite my countless hours of actual study/tutor/homework time invested) and I can use all the help I can get! By the way, it is just a principles of accounting class... The problem (for me) is that it is a 2nd summer semester class. If I wasn't working full time already and IF the instructor had a heart (or taught at a "principles" level rather than testing at a tenured banker level), the exam averages wouldn't be 63's. :(

Quote from: WL2
TRAILHEAD        ,  29.261040,-103.369340
SIGN IN BOX     ,  29.260500,-103.374490
CORRAL            ,  29.268930,-103.422390
STONE HOUSE   ,  29.270390,-103.423010
CREEK CROSS 1,  29.268770,-103.421810
MATATE             ,  29.270280,-103.420610
CREEK CROSS 2,  29.273480,-103.419680
SIGN                ,  29.275059,-103.419915
CANMAX05        ,  29.277695,-103.421380
« Last Edit: March 27, 2008, 04:08:30 PM by RichardM »
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Offline badknees

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GPS coordinates question
« Reply #1 on: August 11, 2007, 12:36:30 PM »
Quote
I'm not understanding how these coordinates work


Make sure you are entering in the same format as your GPS is set up. Or change the format in the GPS.

Look on p52 of your manual at the topic listed POSITION FORMAT

In the words of a famous American - RTFM
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PS. Looks like a pretty nice unit
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Offline presidio

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GPS coordinates question
« Reply #2 on: August 11, 2007, 02:41:48 PM »
Quote from: "stingrey"
I'm not understanding how these coordinates work... The long string of #s doesn't seem to work properly...

I'm sure the info works and I just haven't spent enough time w/my new toy, but does anyone have a quick clue for me?

Quote from: "WL2"

TRAILHEAD        ,  29.261040,-103.369340
...


Quick primer...
a) The coordinates shown are in latitude and longitude; the other common coordinate system is UTM...however, every GPS I've ever picked up required you to go into setup and manually change to UTM, so your GPS should be in the Lat/Long system by default unless you've twiddled with it and don't remember changing the mode. I suspect your unit is in (see below) the DMS mode and you are trying to enter the decimal data shown in the other post, and the example point above.)

b) Back to Lat/Long.

There are 3 modes for Lat/Long data. Your GPS doesn't care which mode it's in, but you have to understand the data formats, otherwise you will not get accurate results.

The traditional one, and the one your GPS probably was shipped with as the default is the degrees, minutes, seconds.decimal seconds. This is shorthanded by ddd mm ss.s

If you are using a paper map and trying to interpolate positions, this is by far the hardest way to do it as you must think in intervals of 360 degrees, 60 minutes, 60 seconds and fractions of seconds. Very difficult to do manually with any precision. Again, your GPS is not bothered by this and if you have all the data in this form it is easy to put into the GPS.

The data would look something like this (omitting the coordinate marks): 104 30 30.00 for a longitude. Depending on your GPS you will either have to put a dash (minus sign) in front of the longitude to tell the unit you are dealing with a 'west' coordinate (otherwise your point will be on the other side of the world) or you must also select 'west' in the data window on your unit.

The next variation on Lat/Long is the degrees, minutes.decimal minutes. This is shorthanded by ddd mm.m. Notice there are no longer any seconds to deal with as they have become a decimal component of minutes. The data would look like this 104 30.50. For purposes of comparison, this is the same coordinate at the one listed under DMS, but it does look different, doesn't it? I am holding the decimal to two places, but as you have seen in the posted coordinates they run to 6 places. Using this system manually is slightly easier to interpolate since the seconds have dropped out, but it still is cumbersome to use on paper.

The easiest variation is degrees.decimal degrees. This is shorthanded by ddd.dd. No minutes or seconds now. The above initial coordinate would now look like this: 104.50, or in the style of the listed coordinates.... 104.500000. Much easier to use manually on paper as you are fully in the decimal system and can easily plot interpolations. In this regard it is about as easy to plot as UTM, but UTM gives you better accuracy as you are down to the meter increment in that system versus a variable distance in Lat/Long depending on where you are north of the equator.

Again, your GPS doesn't care about any of this, it just needs the data in the correct format for the mode shown. You can go into setup and select your preferred mode, or go change it temporarily to accommodate someone else's data so you don't have to perform manual conversions before entering it.

Now, having said all this, if you are in fact in the DMS mode, you don't really have to change anything to enter decimal degree data. Just put it all under the degree window (being sure to insert the decimal point) and make sure the minute and second windows are either blank or have zeros in them. The GPS will know what to do with the decimal data and when you next look at it as a waypoint it will show up as properly converted DMS data.

The only other thing to be aware of is having the proper datum. GPS units default for the display to WGS84 (the same as NAD83, which some units will have). Most people never tinker with this so most of the data you get from someone else will be in WGS84 (and REGARDLESS of the display format, if they have output the data to a file and sent that to you, it WILL be in the WGS84 datum as that is the only way data is STORED in the GPS).

Now, if you have plotted a point manually on a map and have been able to finely determine the coordinates, that is a horse of a different color. With some exceptions, and likely none affecting Big Bend at this point, maps are printed in the NAD27 datum. If you have NAD27 data, you must go into your GPS and select that datum BEFORE you start adding points, otherwise you will be off (and sometimes WAY off) from where you think you are.

Once the data is in the GPS you can either go back to WGS84 or leave it in NAD27. And, again, the GPS doesn't care; all that NAD27 data you entered was immediately converted internally to the WGS84 datum.

Since you are apparently new to GPS and navigation, you would be well served by going through the manual and trying every feature. If you get too confused, you should be able to reset the whole thing to the default shipping settings. If you do that, any points you care about in the memory will be erased, so be sure that you can tolerate the loss. If your GPS has a USB cable, you can download all the data to your computer (and upload). If you are dealing with a lot of data it is far more efficient to enter it this way than laboriously punching in individual coordinates, with the potential for error at every key press.
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