# Degrees ?

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Which one can you get closer to a mark with.

hddd.ddddd

hddd mm.mmm'

hddd mm'ss.s"

What does each one stand for. I am confused!

Thanks

1) Theoretically they all have equal accuracy -- and more accuracy than your GPS. Practically, cache locations are given in DDD MM.MMM, so that's the way to go.

2) The Ds mean degrees of latitude or longitude; the Ms mean minutes; and the Ss mean seconds. Therefore DDD MM.MMM is a latitude or longitude given in degrees, minutes, and parts of a minute. DDD MM SS.SSS is a latitude or longitude given in degrees, minutes, and seconds.

3) What is really important is to make sure your GPS is using the same map datum as the cache location (WGS 84). If it isn't, you could be off up to several hundred feet. The manual will tell you how to check and adjust it

One thing to learn is how to use UTM's because they can be plotted on a map.

http://www.maptools.com/UsingUTM/index.html

Tahosa - Dweller of the moutain tops.

quote:
Originally posted by Brown Dwarf:

1) Theoretically they all have equal accuracy -- and more accuracy than your GPS.

That's not entirely correct. Coordinate systems in and of themselves have no accuracy. What they have is precision. With this in mind, Decimal Degrees is the most precise of the three, (down to about 3.5 feet) followed by Decimal Minutes, (~6 feet) and finally Sexagesimal (aka Degrees) with a precision of about 10 feet. Your point is a good one though, all these are more precise than the stated accuracy of the GPS: 49 feet SIS.

In other words, since the precision of all these coordinate systems is less than the accuracy of the system, it does not matter which one is used, your ability to find geocaches will not be affected.

I also agree that since geocaching.com uses Decimal Minutes, this is the one to use. There is no point in converting this unless you need Sexagesimal or UTM for map use.

As stated previously, I would advise "truckon" to insure his GPS is set for Datum WGS-84 and Coordinate System Decimal Minutes before manually inputting coordinates. If this is not done, one would likely be quite a ways from the intended coordinates.

One final note: If you use easy GPS for input of caches into your GPS, all of this is moot as it does the conversion for you. In this case, it does not matter what you have the GPS set on, you'll still get close enough.

Thanks,

Scott / Brokenwing

http://www.cordianet.com/geocaching

Quote from Brokenwing

That's not entirely correct. Coordinate systems in and of themselves have no accuracy.

That's not entirely correct. Calculations accuracy will vary depending on which system you are using. For geocaching, as it is today, this means nothing. For surveying or navigation that is another story.

For example:

1. Convert to UTM and find the distance and direction between points N 45º00’ W 075º00’ and N 45º01’ W 075º00’.

2. Now convert to UTM and find the distance and direction between points N 45º00’ W 078º00’ and N 45º01’ W 078º00’.

3. Notice the difference in the two results.

4. The answer for step one will have the same numbers if you change latitude to W 081º, W087º, or add (or subtract) any multiple of six degrees.

5. The answer for step two will have the same numbers if you change latitude to W 084º, W090º, or add (or subtract) any multiple of six degrees.

6. This would not occur if you used one of the more complicated, and more accurate Lat/Long calculations.

7. UTM is a flat (or at best cylindrical) system. IT LACKS THE ACCURACY. It has bumps every six degrees of longitude.

8. If you like you may repeat these calculation at another latitudes. It will be interesting.

WWW, your missing some basic points with coordinate systems (better called formats) in that they are all direct mathematical transformations/conversions based on the underlying cartesian cordinate system, which GPS is based on.

Once one starts talking distances then the same "type" of distance (all distance types aren't the same) will be no different based on the same physical locations regardless of the method used to derive that distance (which needs to be specified). The relationship between plane, spheroidal type distances etc are well defined.

Cheers, Kerry.

I never get lost everybody keeps telling me where to go

Kerry, you refer to the "underlying Cartesian coordinate system, which GPS is based on." I wish you would explain this more thoroughly. To me you are saying that GPS is based on a planar or flat earth model. Strange, I had always read and heard that since Clarke in the 1860s an ellipsoid model was used. Also that the Clarke, NAD 27, and WGS 84 were ellipsoid models very close to a sphere.

I also believe you do not understand the purpose asking you to look at the differences between points N 45º00’ W 075º00’ and N 45º01’ W 075º00’ and compare them to points N 45º00’ W 078º00’ and N 45º01’ W 078º00’. The purpose was:

A. True north is usually defined as the direction to the North pole. The shortest way is along a line of constant longitude. At N 45º00’ W 078º00’ the UTM north (sometimes called grid north) differs from longitudinal north by 2.1º. This 2.1 degrees is doubled if you go from zone 18 to zone 17.

B. These jumps or inaccuracies in both length and direction may cause problems in precision surveying or navigation.

quote:
Originally posted by Waterboy With Wife (www):

Kerry, you refer to the "underlying Cartesian coordinate system, which GPS is based on." I wish you would explain this more thoroughly. To me you are saying that GPS is based on a planar or flat earth model.

"Cartesian" doesn't necessarily mean planar. You can have 3D Cartesian coordinates, too: X, Y, and Z. Your GPS receiver figures out your location in a Cartesian space, then projects it onto your chosen ellipsoid.

Deals with the Shuttles GPSr and Right Ascention and Declination.

Tetrahedron in a sphere=GEOID=Perfect Pyramid. On all Planets there is a Geometric disturbance at 19.5 degrees!!!

[This message was edited by Trailblazer # 1 on October 24, 2002 at 08:01 PM.]

quote:
Originally posted by Waterboy With Wife (www):

I also believe you do not understand the purpose asking you to look at the differences between points N 45º00’ W 075º00’ and N 45º01’ W 075º00’ and compare them to points N 45º00’ W 078º00’ and N 45º01’ W 078º00’. The purpose was:

A. True north is usually defined as the direction to the North pole. The shortest way is along a line of constant longitude. At N 45º00’ W 078º00’ the UTM north (sometimes called grid north) differs from longitudinal north by 2.1º. This 2.1 degrees is doubled if you go from zone 18 to zone 17.

B. These jumps or inaccuracies in both length and direction may cause problems in precision surveying or navigation.

Your point is well taken, but it seems to have little to do with my original post which was that coordinate systems in and of themselves have no accuracy component. Coordinate systems are simply a way to represent a given point. As they are a system of measurement, there is no accuracy involved. You can say that a particular measurement is more or less accurate (as I suppose you intended above), but that does not affect the system itself, only the individual measurements involved.

From your usage above, I think you might also be confusing accuracy and precision. They mean different things. In this instance, accuracy refers to conformity to a specific standard. Since the coordinate system in use IS the standard, it’s impossible to say that one system is more accurate than any another. They are all simply different ways to measure the same thing.

Look at it like this: If I measure a line twice, once using a standard metric ruler, and once using an english ruler, (the systems) I will get two different numbers. The line is still the same length (just as a point in space is still the same point). To say that one measurement was more accurate than another simply based on the system in use is incorrect. Accuracy does not come into play between the systems, only between the measurements. As an example, it’s likely that one of my measurements using one of the rulers was more accurate than the other. This is again not a function of the measurement system, but of the measurement process itself.

In the example here, as with measurement of points in space, precision refers to the degree of refinement with which a number can be represented. For example, 3.1242 is more precise than 3.124. The remainder of my original point was that some systems are more precise than others just as the metric system is more precise than the english system in measuring linear distance. This is a function of the scale used for the measurement and has nothing to do with accuracy directly. Precision can allow for greater accuracy, but you can also have a very precise, very inaccurate measurement.

Scott / Brokenwing

http://www.cordianet.com/geocaching

quote:
Originally posted by Waterboy With Wife (www):

Kerry, you refer to the "underlying Cartesian coordinate system, which GPS is based on." I wish you would explain this more thoroughly. To me you are saying that GPS is based on a planar or flat earth model. Strange, I had always read and heard that since Clarke in the 1860s an ellipsoid model was used. Also that the Clarke, NAD 27, and WGS 84 were ellipsoid models very close to a sphere.

Probably should have correctly referred/termed it as ECEF XYZ (Earth-centred, Earth fixed XYZ) or basically a cartesian system based on a 0,0,0 origin (centre of earth). GPS satellites being in geocentric orbits effectively rotate about this origin. All other "user" friendly GPS formats technically originate from ECEF XYZ.

There's no real resemblance between Clarke (which one?) and WGS84 and NAD27 (and others) in geodetic terms 'cause if there was then there wouldn't be a need to have around 200 datums in most GPSr's to cover different parts of the world. Even with NAD27 some receivers have 11 "different" definitions (of the same datum!).

quote:

B. These jumps or inaccuracies in both length and direction may cause problems in precision surveying or navigation.

Not to a surveyor it doesn't besides what's the problem.

Cheers, Kerry.

I never get lost everybody keeps telling me where to go

Is the common term for the number of decimals "precision", not "resolution"?

Sorry for bothering you about this, but I don't speak English unless I have to, so it's interesting to learn what words are usually used for this and that. To me, precision sounds very much like accuracy, but resolution is just the question about how many digits you use to define a number.

Anders

quote:
Originally posted by Brokenwing:

The remainder of my original point was that some systems are more precise than others just as the metric system is more precise than the english system in measuring linear distance.

I was with you right up to here. The metric system is not more precise than the english system in measuring linear distance.

I have here a brand-new bright plastic 30-cm ruler that's graduated in millimeters (with marks that are almost half a millimeter wide) and the 12 inch steel rule from a lovely old Starrett combination square that's graduated in sixty-fourths of an inch (with finely engraved lines.) Which one is more precise again? (Hint: 25.4 millimeters = 1 inch.)

I think the point you're trying to make is that millimeters allow for more precision than sixteenths of an inch, since those are the graduations we're used to seeing on the el-cheapo plastic rulers we all had in school. That is true, but again it's not a comparison of the systems but of the measurements themselves.

quote:
Originally posted by Kerry:

There's no real resemblance between Clarke (which one?) and WGS84 and NAD27 (and others) in geodetic terms 'cause if there was then there wouldn't be a need to have around 200 datums in most GPSr's to cover different parts of the world.

Gosh, I hope there's a resemblance between Clarke (let's pick 1866; that's a nice round number) and NAD 1927, since NAD 1927 is based on the Clarke 1866 ellipsoid.

quote:

Even with NAD27 some receivers have 11 "different" definitions (of the same datum!).

The reason for this has nothing to do with NAD 1927 as a datum and everything to do with the imprecisions inherent in the conversion formulae. By splitting the continent into various regions, you can optimize the conversion factors for a smaller area and get less overall distortion. If everything you were doing were in NAD 1927 - for example, if you were surveying in the year 1945 - you wouldn't need to split up the continent.

quote:
Originally posted by Warm Fuzzies - Fuzzy:

I think the point you're trying to make is that millimeters allow for more precision than sixteenths of an inch, since those are the graduations we're used to seeing on the el-cheapo plastic rulers we all had in school. That is true, but again it's not a comparison of the systems but of the measurements themselves.

You are correct. Sorry for the confusion. I meant to say "can be" and not "is". I was indeed thinking about the typical scale used on a standard 1/16 in ruler in comparison to a mm scale ruler. Thanks for the correction.

Scott / Brokenwing

http://www.cordianet.com/geocaching

edited to correct a spelling mistake...

[This message was edited by Brokenwing on October 25, 2002 at 01:33 PM.]

quote:
Originally posted by Anders:

Is the common term for the number of decimals "precision", not "resolution"?

Sorry for bothering you about this, but I don't speak English unless I have to, so it's interesting to learn what words are usually used for this and that. To me, precision sounds very much like accuracy, but resolution is just the question about how many digits you use to define a number.

Anders

Precision would be more correct (more precise?), but I'll let you draw your own conclusions:

From Marriam-Webster's Collegiate Dictionary: (unrelated definitions eliminated for clarity and conciseness)

res·o·lu·tion

1: the act or process of reducing to simpler form: as a: the act of analyzing a complex notion into simpler ones

pre·ci·sion

1: the quality or state of being precise

2: the degree of refinement with which an operation is performed or a measurement stated

ac·cu·ra·cy

1: freedom from mistake or error

2: conformity to truth or to a standard or model

Does this help?

Thanks,

Scott / Brokenwing

http://www.cordianet.com/geocaching

I've been reading this with great interest and confusion. Here's how I'm understanding it:

If you stand at N0.00 W0.00...well, you'd drown, but ignoring that restriction...and your GPSr says N0.00 W0.00 every time, it is precise and accurate. If it says N0.02 W0.00 every time, it is precise but inaccurate. If it varies by N+/-.02 but averages N0.00 W0.00, it is accurate but imprecise.

Is this correct?

quote:
Originally posted by Brokenwing:

_res·o·lu·tion_

1: the act or process of reducing to simpler form: as a: the act of analyzing a complex notion into simpler ones

_pre·ci·sion_

1: the quality or state of being precise

2: the degree of refinement with which an operation is performed or a measurement stated

_ac·cu·ra·cy_

1: freedom from mistake or error

2: conformity to truth or to a standard or model

Interesting. Let me take a shot at this:

Resolution, in this context would refer to the fineness of actually detrmining your position, that is the actual detail of knowledge.

Precision would refer to the level of detail you

use to express that knowledge.

Accuracy would refer to how correct that knowledge is.

An example: Let's say you were standing exactly at the corner of 1st Ave. and Main St.

A hypothetical positioning system (the whole system...not just a receiver) with a very low resolution might determine you were somewhere near the corner of 1st Ave. and Main streets, but could not determine if you were right on the mark or 100 yards away from the mark.

A system with a very high resolution might be able to determine if you were right on the mark or .001 inch from the mark.

In either case, the reciever your holding might give a very low precision reading, saying "near 1st and Main" or a very high precision reading saying "00.000 inches north of 1st and Main.

The information is correct, for the limits of the resoution of the system and precision of the system.

If you got either of those readings while you were actually standing 3 miles away on Elm St and 10th Ave., the system still has the same resolution and precision, it's just not at all accurate.

How's that?

ApK

quote:
Originally posted by Dinoprophet:

I've been reading this with great interest and confusion. Here's how I'm understanding it:

Is this correct?

Send it to the extremes:

If you're standing at N 00° 01.000 W 000° 01.000, you would be 1 minute north and west of the intersection of the Prime Meridian and the Equator.

If your GPS says N 0° E 0°, it is accurate, but not precise - no minutes, no trailing decimals.

On the other hand, if your GPS says N 43° 30.011 W 084° 45.241, it is precise, but not very accurate (it's telling you you're in the Lower Penisula of Michigan).

Markwell

Chicago Geocaching

quote:
Originally posted by Warm Fuzzies - Fuzzy:

Gosh, I hope there's a resemblance between Clarke (let's pick 1866; that's a nice round number) and NAD 1927, since NAD 1927 is based on the Clarke 1866 ellipsoid.

Resemblence yeah probably wrong word to use. Now the "Clarke 1866" ellipsoid and NAD27 datum do relate. Maybe I should have said Clarke (either 1866 or 1880) is an ellipsoid, where as NAD27 is a datum same as WGS84 but the WGS84 datum is based on the WGS84 ellipsoid and they're different things in geodetic terms. NAD27 is not an ellipsoid (as such).

quote:

The reason for this has nothing to do with NAD 1927 as a datum and everything to do with the imprecisions inherent in the conversion formulae. By splitting the continent into various regions, you can optimize the conversion factors for a smaller area and get less overall distortion. If everything you were doing were in NAD 1927 - for example, if you were surveying in the year 1945 - you wouldn't need to split up the continent.

But I wouldn't call it "imprecisions inherent in the conversion formulae" as the problem with NAD27 stems back over several hundred years with the base data and equipment accuracies all rolled up into the one system. (maybe you meant the same thing?) In 1945 surveyors were still adding to the problem but didn't have the capability in those days to do anything about it.

Similar sort of thing with AGD66 & AGD84 (Australian Geodetic Datum 66 & 84) both based on the same ellipsoid but different coordinate origins due to "additional information" and "strength" in the base data.

Becomes a bit nasty and fragmented with overlapping regions.

The quicker there's a uniform world wide compatible datum the better.

Cheers, Kerry.

I never get lost everybody keeps telling me where to go

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