GPS Formats

[kc2iyh]

Which one is more accurate? dd mm.dec, deg mm ss or UTM?

[robertlipe]

Which is more accurate, Metric or SAE? Oh, they're different ways of representing the "same" thing, you say? Likewise.

 

OK, since you're just down the road from me ("Hi", from Franklin...) I'll try to be a little more helpful. The first are are just very straightforward conversions from one to the other. Which is more accurate: 45 seconds or 3/4 of a minute? That's actually a comparable analogy to the first two. The UTM thing is a little more warped as it tries to break the planet into chunks of the earth small enough to consider flat. As you might guess (assuming you buy into the whole "the earth is NOT flat" thing) there are some distortions involved if you attempt that for a non-trivially sized chunk of the planet.

 

If you have a single point that's represented accurately in all three of the above, none is inherently more accurate.

[kc2iyh]

Which is more accurate, Metric or SAE? Oh, they're different ways of representing the "same" thing, you say? Likewise.

 

OK, since you're just down the road from me ("Hi", from Franklin...) I'll try to be a little more helpful. The first are are just very straightforward conversions from one to the other. Which is more accurate: 45 seconds or 3/4 of a minute? That's actually a comparable analogy to the first two. The UTM thing is a little more warped as it tries to break the planet into chunks of the earth small enough to consider flat. As you might guess (assuming you buy into the whole "the earth is NOT flat" thing) there are some distortions involved if you attempt that for a non-trivially sized chunk of the planet.

 

If you have a single point that's represented accurately in all three of the above, none is inherently more accurate.

 

Wow, thats what I thought but someone once debated that with them adamantly that dd mm.dec was the second most accurate and UTM was more. I was not sure how UTM worked so I had nothing to offer there but was under the impression the difference between dd mm ss and dd mm.dec was just simply a conversion. Thanks for clearing that up and hello from Columbia. See you on the GC hunts.

[Suscrofa]

Accuracy has nothing to do with the measurment units used, just the equipment and methodology !

 

Now a coherent system will likely be more accurate because of lack of conversion and rounding. In metric you have only one unit per type of measure (meter, kilogram, second and ampere), (kilometer, millimeter are not different unit, kilo means thousand, milli 1/1000).

This ease of use usually means that people will tend to express a measure more precisely.

 

In the US system, you have different unit depending on the size (inch, feet, fathom, yard, rod, mile ...), area, volume, even the the type of material (dry, liquid, grain, precious metal)

All this involves different conversion factor, some very strange, lot of confusion, prone to rounding errors if not gross errors.

 

Just take a ruler, if in metric, you will give the measure in mm if not half mm while in inch you will tend to round it to what ? 1/16, 1/32, tenth of inch ??

 

Look at maps, on US maps, the scale is one inch equals approximately that much feet or yards or miles. On regular map, no such thing, scale 1/100 000, simple, one cm is one km, 1/25000 one cm is 250m !

Even with US topo maps scale 1/24000, one cm being 240m, metric is easier than customary. It was the way I was using these maps while in the US.

 

This remind me the shame of a civil engineering teacher who used to build bridges in India (in the fifties) and had one of his bridge collapse because of this kind of confusion on the steel rod sizes for the concrete (stupid british gauge system !) when it was put in service in front of the top authorities. Gee, he hated this system !

[draselok]

UTM is ideal if you are using TOPOs with your GPS unit. A standard TOPO has 1 Km sq grid marked out on it.

Setting your GPS to UTM allows you to pinpoint, within 25m where you are on the map. That is extreamly

difficult if you are working in lat/lon.

 

Here is good link to using UTM http://www.maptools.com/UsingUTM/

[+The Forester]

I think you are asking about resolution, not "accuracy".

 

The GC.com standard format of DD MM.mmm produces a Latitudinal resolution of 1/1000th of a nautical mile, ie 1.852m, and a Longitudinal resolution which varies as the Cosine of the Latitude. At Latitude 60° the Longitudinal resolution therefore becomes Cos60 * 1.852 = .5 * 1.852 = 0.926m. Taking the diagonal by applying Pythogoras gives us a resolution of 2.071m.

 

The single metre resolution of UTM, when applying Pythogoras to find the resolution of both Eastings and Northings gives a resultant resolution which is the square root of 2, ie 1.414m.

 

Therefore you can see that the resolution of UTM grid co-ords is slightly finer than the DDMM.mmm format.

 

The DDMMSS format is much coarser resolution, as an arcsecond of Latitude is a 60th of a Nautical Mile, which is about 30.867m.

 

If you want to choose between the various options on your GPSr of ways to display co-ords to the finest resolution, for example so that you can write down lots of fixes for later averaging, pick UTM as your best option. Once you've averaged your list of co-ords, change back to the GC.com format of Degrees and three decimal places of Minutes.

[John E Cache]

If you want to choose between the various options on your GPSr of ways to display co-ords to the finest resolution, for example so that you can write down lots of fixes for later averaging, pick UTM as your best option. Once you've averaged your list of co-ords, change back to the GC.com format of Degrees and three decimal places of Minutes.

How did you figure that out? With experiments? I was thinking, like a calculator, the numbers stored internally may have more significant digits than the display. Also, the internal number format might not change when you change the display format.