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Papa-Bear-NYC

U T M (universal Tansverse Mercator)

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There was a recent thread on the difference between NAD83 and WGS84.

 

I have some questions on UTM in case there are any experts here. I actually prefer it to other projections used for maps since it's intuitive to think of so many meters north or east. Furthermore, maps that have the UTM grid lines superposed on them are very nice since you see 1 km blocks and it's easy to see distances without reference to the scale.

 

On a typical Topozone page, at the bottom it says "Projection is UTM Zone 18 NAD83 Datum".

 

My questions:

 

Is UTM (as implied by the above) simply a map projection as opposed to a Datum?

 

In high school (I think) I learned that a Mercator projection was "rectilinear" in the sense that it locally assumed a flat earth, and angles betreen directions were preserved (i.e. right angle in the world get projected as right angls on the map). Is this true, or just a high school simplification?

 

Is it always tied to NAD83? I though UTM was invented years ago by the military and so would be tied to WSG84.

 

Is there a control issue? Since these maps are produced (before Topozone gets them) by the USGS, I assume the maps are controlled by USGS monuments (which I assume use NAD83).

 

Is the center of the zone a control point?

 

Is there measurable distortion at the edges of a zone?

 

Links to explain this stuff would be appreciated.

Edited by Papa-Bear-NYC

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PBN:

Wow. Lots of questions.

Courtesy of your taxpayer dollars (thanks!) and our US Military, I have a decent working knowledge of UTM. The military calls it MGRS (or "Grid" for short). There are some minor differences.

 

There are lots of websites that explain it fairly well.

Maptools is one of my favorites, when I need to re-remember something. They make a nice booklet of the stuff on their web also.

 

There is a USGS webpage for it also.

 

In a quick nutshell:

Mercator is a type of map projection (named after guess who?)

[A projection being the process to go from round earth to flat paper).

 

Transverse Mercator means that the projection is across the equator (i.e. North to South). Opposite what you have seen before.

 

Universal Transverse Mercator (UTM) is a coordinate system, serving the same "function" as Latitude & Longitude, but different. It is not a datum. UTM coordinates can be in any (horizontal) datum.

 

It's good stuff. I try to use it, but Geocaching & Benchmarking have swung me more to the Lat / Long camp, unfortunately.

Edited by Klemmer & TeddyBearMama

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After you have wrapped you mind around UTM, check out the US National Grid (USNG) System.

 

ESRI has a Tutorial about it also that I have been through once.

 

I'm not so sure when or if that will ever catch on. It is essentially an extension (or simplification?) of UTM. Lots of taxpayer money paying for USNG at the FGDC. I wonder if you will ever see USNG coordinates on NGS Datasheets?

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After you have wrapped you mind around UTM, check out the US National Grid (USNG) System.

...

 

I wonder if you will ever see USNG coordinates on NGS Datasheets?

 

You mean like this:

 

KU1301

KU1301_U.S. NATIONAL GRID SPATIAL ADDRESS: 18TWL873048(NAD 83)

KU1301_MARKER: DB = BENCH MARK DISK

KU1301_SETTING: 36 = SET IN A MASSIVE STRUCTURE

KU1301_SP_SET: BUILDING

KU1301_STAMPING: F 351 1952

 

Gotcha!

 

Already there. Every datasheet has it.

Edited by Papa-Bear-NYC

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Re: rectilinear. A map that preserves the angles near a point is called "conformal", and the Transverse Mercator is confomal. That doesn't mean that the directions at large distances are correct, it just means that there is no distortion of the angles between nearby points. That's one of the things that makes it useful as a navigational map. The other thing is that in the Standard Mercator (but not the UTM), all lines of constant bearing (rhumb lines) are straight lines, which makes it easy to plot a course that is held to a constant compass bearing.

 

As far as the term "rectilinear" goes, I suspect that might refer to the fact that latitude and longitude lines are straight and at right angles in the Standard Mercator projection. That is not true of the Transverse Mercator, where lines of latitude and longitude are curved.

 

Re: distortion. The UTM actually has two vertical lines on which there is no distortion, and is increasingly distorted at the center and edges. At the center of each zone, the scale designed to be 0.9996 of the true scale. Since the scale increases toward the edges of the zone, that means there are two lines on each side of the center where the scale is exactly true, about 180 km east and west of the zone center.

 

The center of the zone is simply the center of the zone. It is the one place where a line of longitude is a straight line. Control points are unrelated, and are simply aids to ensure that features are plotted in the correct place on the map. When the landscape is surveyed for mapping, features are located with respect to other features. The control points are simply features that are used to tie the ground truth into the mathematical model of the map projection. Any feature having coordinates that have been accurately surveyed can be a control point.

 

UTM in the U.S. is technically neither NAD83 nor WGS84, since it (by spec) uses the Clarke 1866 ellipsoid, while NAD83 uses the GRS80 ellipsoid, and WGS84 uses WGS84. However, that doesn't preclude anyone from using the Transverse Mercator projection with those datums, it just means that those maps aren't standard UTM.

 

When a map is marked UTM, NAD83 datum, it means that the geographic coordinates of the features have been located using the NAD83 datum, and then plotted on a UTM projection. The latitude and longitude graticule will correspond to the NAD83 datum. The only consequence is that if you measure a point's X and Y coordinates in the UTM grid, and then convert back to latitude and longitude using the formula for the inverse UTM projection, you won't necessarily get the correct NAD83 position unless the map was also made using the GRS80 ellipsoid. In that case the map doesn't conform to the UTM technical standard as written by the Army in 1947. Even if the map was projected using the Clarke 1866 ellipsoid per standard, the error in the calculated latitude and longitude will be quite small.

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To much work in converting for me.

 

I have a theory as well on it.

 

If Thomas Jefferson and the Scientists of the day used the Prime Meridian as base using other functions coordinates or data have to throw error in there.

Commonly used projections

 

Instead of measuring from east to west you go west to east?

 

I think many a man and woman have pondered these same concepts.

That is why there are so many different map projections.

 

Even Clark had his own spheroid.(Clark Spheroid 1866)

[Gdal-dev] OGR vs. ESRI Clarke 1866 Spheroid

MAINE

 

And if you look at details of the old maps many things have changed(been adjusted).

 

High Accuracy Reference Networks (HARN)

For the United States there is an ongoing effort at the state

datum to a higher level of accuracy using state-of-the-art

not widely available when the NAD83 datum was being developed.

readjustment of NAD83. This project, known as the High

(HARN), or High Precision GPS Network (HPGN) is a cooperative

National Geodetic Survey and the individual states.

 

This being due to a new realization of the shape and size of the Earth in a more defined way.

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Q: Is UTM simply a map projection as opposed to a Datum?

 

A: YES. A horizontal geodetic datum consists of an origin, an orientation, a mathematical model of the size and shape of the Earth (e.g. ellipsoid) and a realization (e.g. survey control monuments). UTM is one of many different map projects that are commonly used by national and regional surveying and mapping authorities to "project" geographic cooridates (latitude and longitude) into a more user friendly cartesian coordinate system. Other popular systems are the U.S. State Plane Coordinate System (which uses several different map projections including the Transverse Mercator) and Gauss Kruger.

 

Q: Is it always tied to NAD83? I though UTM was invented years ago by the military and so would be tied to WSG84.

 

A: NO. UTM can, and has been used with hundreds of different horizontal datums around the world. The primary defining parameters in the conversion algorithm are the size and shape of the reference ellipsoid.

 

Statement: UTM in the U.S. is technically neither NAD83 nor WGS84, since it (by spec) uses the Clarke 1866 ellipsoid

 

This is incorrect. UTM is not speced to any particular ellipsoid. In my personal collection I have the old U.S. military manuals for conversion of lat/long to UTM for the Clarke 1866, Clarke 1880, International 1924, Bessel 1841, Airy 1830, Everest 1830 and Fischer 1960 ellispoids.

 

Q: Is there a control issue? Since these maps are produced (before Topozone gets them) by the USGS, I assume the maps are controlled by USGS monuments (which I assume use NAD83).

 

A: There are several kinds of maps/charts produced by Federal agencies. USGS has responsibility for the internal topographic maps, the Office of Coast Survey at NOAA is responsible for all nautical charts and the National Aeronautical Charting Agency of the Federal Aviation Administration is responsible for the national aeronautical charting program. All of these institutions rely on the definitions of the horizontal and vertical datums defined by the National Geodetic Survey and realized by the control monuments. During the many years of the development of the topographic mapping program by USGS they also put in a considerable number of control monuments to densify lower accuracy control for mapping. USGS does not put marks in the ground anymore and is rapidly trying to get out of the business of providing support for their monuments since they have completed the national mapping program.

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Interesting stuff. At least for some of us. Thanks DaveD.

 

PBN: Yep, got me. Never noticed that before. It has been added since 2000 or so, as it seems all the datasheets on GC.com seem to be missing it. I wonder if it is of any real use?

 

Learning curve here: My Magellan Meridian even suppprts USNG!!

11SMT2043450424 (NAD83) doesn't really do much for me, however. Maybe someday.....

 

Now to unscramble my GPSr again......

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