GPS navigation

[iCe]

I saw a post somewhere and someone was complaining about the author not supplying UTM coordinates. I don't know if it was this board or some other board. Probably this one. Anyway- my GPS (Garmin 76CSx) map datum is set on WGS84. I don't use it for hard core navigation. More like a reference. My compass doesn't have batteries so it's more reliable. Is there any reason to use a different map datum?

[tjavery]

...Is there any reason to use a different map datum?

This is a good question for BK.

I believe that WGS84 is somewhat of the "default" datum for most consumer, public electronic* uses (e.g. GPS units). I believe Google Earth defaults to WGS84 as well as Garmin Mapsource.

* USGS topo maps are NAD27, IIRC. It's just slightly different than WGS84 - but different enough to make a difference.

[presidio]

Is there any reason to use a different map datum?

Yes, in a couple of instances.

a) You are using the GPS in conjunction with a paper map and you want to either transfer a point on the map the GPS, or vice versa. In that case the GPS datum must match the map's. WGS84 is the current standard and the datum the GPS defaults to unless you change it. It also is the datum in which stored data is output (at least in Garmins) regardless of what datum you have selected for viewing. The vast majority of paper 7.5' topographic maps, and the ones for Big Bend have not been revised in years (other than to update physical changes via photogrammetry...the purple additions you see on some maps). These maps use the NAD27 datum (North American Datum 1927). If you scroll through the datums you may see a whole bunch of NAD27s...you want the one that appends CONUS (continental US) to the description.

b) You get coordinates from someone that you want to enter in your GPS. The datums must match. If you ask most users what datum their data is in, expect a blank stare. Most folks have no clue nor do they even know what you're talking about. In that case either look at their GPS settings to be sure, or assume (probably correctly) that it is in WGS84. If they don't know what you are talking about it's unlikely they've changed the datum.

So, let's say you prefer to be in WGS84 but you get info in another datum. Merely change your GPS to the other datum, enter the info and then go back to WGS84. The GPS will correctly display the new information in WGS84. This is tedious for more than a couple of points, so I would suggest getting an output file from the person giving you the data, if you can...not all GPSs have a data-out capability. If they do, you will have a text file that should also show you the datum the data is exported in (should be WGS84, but might not be). Then you merely import the data into your GPS (again assuming you have a unit with data import/export capability), matching datums in the process. All is well.

I stay in WGS84 mode regardless of the paper map datum. The error in position between WGS84 and NAD27 is not extreme but can be on the order of up to several hundred meters, but if you know how to read a map it's easy to determine your approximate location on the paper map. Now, if you are deep in the north woods and have the paper map with you, you probably want to use the paper map datum so you have a reasonable chance of not getting lost via technology, but in most of the west you can see a whole lot farther than the inter-datum error.

I never would buy a GPS that cannot input/output data. They cost a little more than those that don't but the functionality is so much greater. Plus, with a data out capability you can do real time navigation on a laptop on digital copies of the paper maps. Been doing that for years.

However, more important than all the gee whiz facets is to know how to read a map and use a compass with it. Unfortunately, for a likely goodly number of folks a GPS is a lot like a flashlight, you turn it on and it works but when the batteries fail you are left in the dark.

[iCe]

Awesome replies. Exactly what I needed.

I have been using a compass and map for years (though not so much recently). Thankfully the compass datum syncs automatically to the datum that the map is derived from. What a great design 

[badknees]

Awesome replies. Exactly what I needed.

I have been using a compass and map for years (though not so much recently). Thankfully the compass datum syncs automatically to the datum that the map is derived from. What a great design 

Presidio has done a good job explaining the "datum" issue. Don't forget to take into account the magnetic deviation when using a compass.

[iCe]

Yep... I learned about that back in private pilot ground school.  In some places deviation is HUGE.

[WL2]

A good thread so far, but the UTM was not explained.  UTM datum uses a value in 'meters' rather than degrees.  This can be useful in some situations and was probably what the original author was talking about.

[badknees]

A good thread so far, but the UTM was not explained.  UTM datum uses a value in 'meters' rather than degrees.  This can be useful in some situations and was probably what the original author was talking about.

UTM is not actually a datum, it is a projection. It stands for Universal Transverse Mercator, which is a cylindical projection of a metric grid on an "spherical" globe. A datum is a model used for calculating the location of the Earth's center. This center calculation is used as a reference point in establishing coordinate locations, such as latitude and longitude, on the Earth's surface.
The globe is not a sphere or even a perfect ellipsoid. Datums are a mathematical description of the shape of the globe. Some datums work better than others describing the globe depending on your location. UTM is a metric grid laid down on a specific datum. WGS84 and NAD83 are the most modern of the common datums used to describe the earth geometry. They are based on the GRS 80 datum which uses an equatorial radius of 6,378,137 meters and a polar radius of  6,356,752.3141 meters. The metric system, which is the basis for UTM, was set up so that the distance from equator to pole would be exactly 10,000 kilometers. (but it is actually about 10,002 km). UTM grids plot differently for each datum, so the datum used must be noted as well as the UTM coordinates. In other words, a NAD 27 UTM coordinate, will not plot to the same location on a WGS84 referenced map.

This is a fairly complicated subject and can't easily be described with my limited typing patience, however if you really are interested in the detail, try this:

Map Projections - A Working Manual. U.S. Geological Survey Professional Paper 1395

Fortunately WGS84 and NAD 83 are virtually identical for our purposes as hikers, but if your are the military targeting a smart bomb you might want to be more accurate. (unless it's a REALLY big bomb).

As far as UTM or Lat/Lon, it really is a personal preference. There is not tangible advantage for accuracy at normal latitudes for recreational users , however I believe a different projection must be used at the poles cause UTM won't work there.

[Al]

A good thread so far, but the UTM was not explained.  UTM datum uses a value in 'meters' rather than degrees.  This can be useful in some situations and was probably what the original author was talking about.

UTM is not actually a datum, it is a projection. It stands for Universal Transverse Mercator, which is a cylindical projection of a metric grid on an "spherical" globe. A datum is a model used for calculating the location of the Earth's center. This center calculation is used as a reference point in establishing coordinate locations, such as latitude and longitude, on the Earth's surface.
The globe is not a sphere or even a perfect ellipsoid. Datums are a mathematical description of the shape of the globe. Some datums work better than others describing the globe depending on your location. UTM is a metric grid laid down on a specific datum. WGS84 and NAD83 are the most modern of the common datums used to describe the earth geometry. They are based on the GRS 80 datum which uses an equatorial radius of 6,378,137 meters and a polar radius of  6,356,752.3141 meters. The metric system, which is the basis for UTM, was set up so that the distance from equator to pole would be exactly 10,000 kilometers. (but it is actually about 10,002 km). UTM grids plot differently for each datum, so the datum used must be noted as well as the UTM coordinates. In other words, a NAD 27 UTM coordinate, will not plot to the same location on a WGS84 referenced map.

This is a fairly complicated subject and can't easily be described with my limited typing patience, however if you really are interested in the detail, try this:

Map Projections - A Working Manual. U.S. Geological Survey Professional Paper 1395

Fortunately WGS84 and NAD 83 are virtually identical for our purposes as hikers, but if your are the military targeting a smart bomb you might want to be more accurate. (unless it's a REALLY big bomb).

As far as UTM or Lat/Lon, it really is a personal preference. There is not tangible advantage for accuracy at normal latitudes for recreational users , however I believe a different projection must be used at the poles cause UTM won't work there.

bk, wow! Although familiar with the UTM coordinate system and its use generally as well as the metric system, I did not know that the meter is based on the calculated distance from the Equator to the pole.  VERY educational.  Thanks!

Al

[presidio]

The metric system, which is the basis for UTM, was set up so that the distance from equator to pole would be exactly 10,000 kilometers.

And for those who might be curious, the coordinates are expressed in units of 'northings' and 'eastings'. The northing value is an expression of how many actual meters you are north or south of the equator. The easting is a different animal. Each UTM zone has a base unit of 500,000 assigned to the center of the zone. Your position within each zone thus is greater or lesser (or equal to if on the line) than 500,000. What this does is prevent any negative numbers from being created by a position reading. The nominal spread is about 680,000 meters across any UTM zone, thus you typically would see an extreme low value on the west side of a zone in the vicinity of 160,000 and an extreme high value on the east side of the zone of around 840,000. The resultant values of position are called 'false eastings'. This has no implication for using a GPS, just background on how the position is determined.

As far as UTM or Lat/Lon, it really is a personal preference. There is not tangible advantage for accuracy at normal latitudes for recreational users

Not accuracy, perhaps, but from a standpoint of convenience, UTM is the easier to use. Since Lat/Long consists of degrees, minutes and seconds and is a coordinate system laid on a sphere, the lines of longitude converge at the poles. The lines of latitude between them vary in length, becoming progressively shorter as you move away from the equator. Thus, while length of a second of longitude can be considered as a fixed value, the length of a second of latitude depends on where you are on the planet. For example, a second of latitude at the equator is approximately 1.15 miles, which is the same as a nautical mile, and is where the term came from. However, a second of latitude at, say Houston, equals just a hair over a mile. And, at New York City it is only about .87 miles.

This makes direct calculations of distance hard to do easily or quickly (lacking a computer). UTM, on the other hand, being a metric grid with parallel and perpendicular lines makes calculating a distance (at least within the same zone) a breeze. It's simple addition and subtraction to determine how far up and/or over one point is from another. You still need geometry to determine the direct distance, but at least you can get a quick idea of where an unknown point lies.

This is further made easy by using a decimal coordinate system rather than interpolating the DMS of lat/long.

[MilesOfTexas]

However, more important than all the gee whiz facets is to know how to read a map and use a compass with it. Unfortunately, for a likely goodly number of folks a GPS is a lot like a flashlight, you turn it on and it works but when the batteries fail you are left in the dark.

Admittantly, I need the full course for compass navigation.  I learned it when I was in boy scouts some 30 years ago, but for some reason can't remember it, haha.  I always have a compass with me, but if I needed to 'really' use it, it would be a tough go.  Online there are sites that have various 'instructions', but I am wondering if you know of an 'authoritative' publication that I can purchase.

[badknees]

The metric system, which is the basis for UTM, was set up so that the distance from equator to pole would be exactly 10,000 kilometers.

And for those who might be curious, the coordinates are expressed in units of 'northings' and 'eastings'. The northing value is an expression of how many actual meters you are north or south of the equator. The easting is a different animal. Each UTM zone has a base unit of 500,000 assigned to the center of the zone. Your position within each zone thus is greater or lesser (or equal to if on the line) than 500,000. What this does is prevent any negative numbers from being created by a position reading. The nominal spread is about 680,000 meters across any UTM zone, thus you typically would see an extreme low value on the west side of a zone in the vicinity of 160,000 and an extreme high value on the east side of the zone of around 840,000. The resultant values of position are called 'false eastings'. This has no implication for using a GPS, just background on how the position is determined.

As far as UTM or Lat/Lon, it really is a personal preference. There is not tangible advantage for accuracy at normal latitudes for recreational users

Not accuracy, perhaps, but from a standpoint of convenience, UTM is the easier to use. Since Lat/Long consists of degrees, minutes and seconds and is a coordinate system laid on a sphere, the lines of longitude converge at the poles. The lines of latitude between them vary in length, becoming progressively shorter as you move away from the equator. Thus, while length of a second of longitude can be considered as a fixed value, the length of a second of latitude depends on where you are on the planet. For example, a second of latitude at the equator is approximately 1.15 miles, which is the same as a nautical mile, and is where the term came from. However, a second of latitude at, say Houston, equals just a hair over a mile. And, at New York City it is only about .87 miles.

This makes direct calculations of distance hard to do easily or quickly (lacking a computer). UTM, on the other hand, being a metric grid with parallel and perpendicular lines makes calculating a distance (at least within the same zone) a breeze. It's simple addition and subtraction to determine how far up and/or over one point is from another. You still need geometry to determine the direct distance, but at least you can get a quick idea of where an unknown point lies.

This is further made easy by using a decimal coordinate system rather than interpolating the DMS of lat/long.

Although I mostly agree, but have some comments.

1. If you are a LAT/LON person, use decimal degrees, and not DMS - Much easier.

2. UTM requires a Zone designation as well as northing and easting. Additionally there are 2 points on the globe that have the same coordinates and must be differentiated by an additional notation of which hemisphere. Sometimes the convention to identify the southern hemisphere point is to use an "S" and this can cause some confusion because there is also a zone S.

I can't remember the last time I ever used coordinates to mentally estimate distance. In either case as you said geometry is still required unless the coordinates are lying along the same grid.

I'm not against either one.

[jeffblaylock]

Personally, I prefer UTM for the reasons Presidio outlined. When looking down at a custom-made topo map (generated from digital rasters of the USGS quads), I know that each grid line represents 1 kilometer, or 10 football fields with a few end zones thrown in. I have used it to estimate distance. In addition, the fact that each square is 1,000 meters, it is easier for me to eyeball my position than in a LAT/LON world, even when using decimal degrees. It is a matter of taste, as there is no right or wrong answer.

I don't recall a complaint here that something wasn't posted using UTM, but I have received several complaints that I only post UTM coordinates.

[iCe]

I was skimming through posts. Someone posted a trip log and someone else asked why no UTM or something like that.

On another note, I downloaded BaseCamp 3.0.2. I kind of like it it but it's not all that and a bag chips. One thing that was disappointing was finding that it couldn't import files from GPSFileDepot.com. Their "solution" is to buy TOPO US 24k maps for my GPS and then once they are installed their "free" software can use them. At $100.00 (more or less) each (Texas is one map) that's not my preferred solution 

Personally, I prefer UTM for the reasons Presidio outlined. When looking down at a custom-made topo map (generated from digital rasters of the USGS quads), I know that each grid line represents 1 kilometer, or 10 football fields with a few end zones thrown in. I have used it to estimate distance. In addition, the fact that each square is 1,000 meters, it is easier for me to eyeball my position than in a LAT/LON world, even when using decimal degrees. It is a matter of taste, as there is no right or wrong answer.

I don't recall a complaint here that something wasn't posted using UTM, but I have received several complaints that I only post UTM coordinates.

[presidio]

I am wondering if you know of an 'authoritative' publication that I can purchase.

Well, you can go here and get this for a few dollars (I have an ancient version of this...very well written)
http://www.amazon.com/Be-Expert-Map-Compass-Orienteering/dp/0020292651

or

you can go here and get a free ebook
http://www.map-reading.com/

Either one will tell you what you need to know.