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Submitting Gpsr Coords To Ngs?


Cyclometh

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I recovered a benchmark today, and took a GPSr reading with my handheld. I assume it's a bit more accurate than the one on the datasheet because it's set as scaled.

 

Question: Should I submit these coordinates in the recovery report, and if so, what coordinate system should I use?

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It is considered a good thing to include GPS receiver coordinates in a found report for scaled marks. These coordinates are considered by the NGS as part of the location description. They don't change the listed coordinates of the mark.

 

Since the difference between WGS84 and NAD83 is much smaller than the precision of the GPS receivers we currently use, you could use either one, but it is generally considered good form to use NAD83 since that it the datum the NGS officially uses.

 

The NGS and its users prefer the form DDD MM SS.S instead of DDD MM.MMM or other formats.

 

Here is an example.

Edited by Black Dog Trackers
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BDT gave an excellent example! Also, we're told that you can shorten the entry to "HH2 Coordinates", per info in the NGS forum. The "2" indicates that the reading is from a consumer-grade GPS receiver, vs. the $5,000 handheld that reads to a few inches (and makes coffee). :D

 

Also note BDT's use of tact in reporting a find that another group missed on two attempts. This is characteristic of reports to NGS. On GEOCACHING.com, we might say something like, "I don't know how the USPSQD missed this one twice. It's in plain sight."

 

But the NGS report always will say, "RECOVERED AS DESCRIBED".

 

-Paul-

 

 

HV2101 HISTORY - Date Condition Report By

HV2101 HISTORY - UNK MONUMENTED PBPP

HV2101 HISTORY - 1930 GOOD WSSC

HV2101 HISTORY - 1944 GOOD CGS

HV2101 HISTORY - 19950709 MARK NOT FOUND USPSQD

HV2101 HISTORY - 20001106 MARK NOT FOUND USPSQD

HV2101 HISTORY - 20050703 GOOD GEOCAC

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This is something that's been bugging me for a while. Though the perferred form is as described above, my Magellan 210 only gives its maximum accuracy (which has been remarkably good) in decimal degrees. I could do a conversion, but that adds another layer where mistakes might occur. Though I've seen the short form used with decimal degrees, that's apt to trip up someone not expecting it. At the moment, I just add a line at the end listing the decimal coordinates. IMO, getting a result some feet closer trumps proper form but it still bugs me.

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At the moment, I just add a line at the end listing the decimal coordinates. IMO, getting a result some feet closer trumps proper form but it still bugs me.

You are providing a more precise value, but probably not a more accurate value. The factors that limit the accuracy of a consumer handheld receiver are external to the receiver, and the extra precision is somewhat meaningless. It's possible that with a clear, sustained view of multiple satellites and WAAS correction, with no multipath interference, your handheld GPS might conceivably be accurate to within a few meters. The difference between the position stated in dd mm ss.s and dd.ddddd is less than about 1 foot, so either format has ample precision given the inaccuracy of our receivers.

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That would be true, but for the fact that there's no "tenths" digit on the DD MM SS readout! As for accuracy, I've spent quite a bit of time using SA Watch to examine drift and what the unit does for various numbers of satellites and signal conditions. Under good conditions and with WAAS (and I'd never post coordinates when conditions are bad) the unit is consistently within about 7', based on both benchmarks and wander around a point. But, only if you use a readout format that can show it. It will do DD MM.MMM, which is way better than DD MM SS, though still loses a (certainly only theoretical) bit to DD.DDDDD. IMO, the perfect GPSr hasn't been made yet, and when it is, I probably won't be able to afford it :blink:

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I see. The Garmin does show tenths of seconds, so there is little difference between the formats. You could either get a calculator that does the conversion for you with a single function button, or set up a spreadsheet to do it. One or the other is preferable to doing it by hand each time, which almost certainly will result in a blunder at some point in time.

 

It's an interesting question about the likelihood of error: if everyone who uses your dd.ddddd coordinates would each have to convert to another format, then each of them has some probability of doing it wrong. If you can do it for them reliably and correctly, then the total number of errors are reduced. However, if you do make a mistake, then everyone who uses your coordinates will be working with the wrong numbers. It boils down to how often you might make a mistake vs. how often your coordinates might be used.

 

edit: typos, of course.

Edited by holograph
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Fact is, I do like the UGS abbreviated form, though the embarrassment of making a mistake bothers me far more than my normal condition of being peculiar and unreasonable. IMO, technical people work in decimals. Anyway, the very nice little Geocalc calculator program takes care of the conversion fine, and I should just resign myself to triple checking for typos and brain fade before submitting.

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Thanks for all the advice, folks. I switched my GPSr to use NAD83. I'm not sure how to convert the display coordinates to the format the NGS likes, though. How do I convert the format mine uses:

 

N 47 02.584 W 122 50.991

 

To the format the NGS uses? I switched my GPSr and the above coordinates become

 

N 47 02'35.0" W 122 53'59.5"

 

There's a huge number of means to both track and notate coordinates, and it can be a bit overwhelming.

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The safest way to convert while still getting the precision, if you have set a waypoint, is to do the manual conversion from dd.ddddd to dd mm ss.s and then switch the GPS unit to dd mm ss format and see that it has the same result down within one least digit.

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The safest way to convert while still getting the precision, if you have set a waypoint, is to do the manual conversion from dd.ddddd to dd mm ss.s and then switch the GPS unit to dd mm ss format and see that it has the same result down within one least digit.

 

OK, and my last (hopefully) ignorant question- what's the manual process for this? I don't mind a little hot paper and pencil action, I just don't know the rules for how to do the conversion.

 

BTW, thanks to everyone here for all the great help! You guys rock.

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If you are going to do a lot of conversions, the spreadsheet will make life simpler.

 

The conversion is just multiplying the parts after the decimal by 60 minutes per degree and 60 seconds per minute. For example:

 

100.12345 = 100 degrees + 0.12345 * 60 minutes =

100 deg 7.407 min and the 0.407 min = 0.407 * 60 sec

100 deg 7 min 24.4 sec

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If you are going to do a lot of conversions, the spreadsheet will make life simpler.

 

The conversion is just multiplying the parts after the decimal by 60 minutes per degree and 60 seconds per minute. For example:

 

100.12345 = 100 degrees + 0.12345 * 60 minutes =

100 deg 7.407 min and the 0.407 min = 0.407 * 60 sec

100 deg 7 min 24.4 sec

 

Hmm, my math was off.

Edited by Cyclometh
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Here is how I show mine..

 

RL0416

RL0416 STATION RECOVERY (1978)

RL0416

RL0416'RECOVERY NOTE BY US GEOLOGICAL SURVEY 1978

RL0416'MARK NOT FOUND.

RL0416

RL0416 STATION RECOVERY (2002)

RL0416

RL0416'RECOVERY NOTE BY US POWER SQUADRON 2002 (GG)

RL0416'MARK NOT FOUND.

RL0416

RL0416 STATION RECOVERY (2005)

RL0416

RL0416'RECOVERY NOTE BY GEOCACHING 2005 (MPR)

RL0416'NAD 83(1986)- 46 06 30.9 (N) 088 50 43.8 (W) HANDHELD GPS

RL0416'

Edited by Z15
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If you are going to do a lot of conversions, the spreadsheet will make life simpler.

 

The conversion is just multiplying the parts after the decimal by 60 minutes per degree and 60 seconds per minute. For example:

 

100.12345 = 100 degrees + 0.12345 * 60 minutes =

100 deg 7.407 min and the 0.407 min = 0.407 * 60 sec

100 deg 7 min 24.4 sec

 

Hmm, my math was off.

 

Just using one longitude for simplicity's sake, when I try to convert W 47 02.584 I get

 

47 deg 37 min 2.4 sec

 

But that's nowhere near what my GPSr said when I switched its display.

 

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Cyclometh,

 

If your GPS shows 47 02.584, then your GPS is giving you degrees and decimal minutes. All you have to do is multiple the decimal portion by 60 to get the seconds:

 

47 02.584 = 47 degrees, 02 minutes, 60 * .584 seconds = 47 deg 02 min 35.0 sec

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OK, I see where I screwed up. I was doing two conversions, not realizing you were starting from a different coordinate system.

 

So according to my calculations:

 

N 47 02.584 W 122 50.991

 

Comes out to:

 

47d 02m 35.04s W 122d 50m 59.46s N

 

Which is within .04 seconds of my GPS when I switch the displayed coordinates.

 

And I now have my GPS in NAD 83 for all the future, as although there's a difference, it's not enough for mine to detect. I can use it for both Benchmark hunting and Geocaching. :blink:

 

Cyclometh,

 

If your GPS shows 47 02.584, then your GPS is giving you degrees and decimal minutes. All you have to do is multiple the decimal portion by 60 to get the seconds:

 

47 02.584 = 47 degrees, 02 minutes, 60 * .584 seconds = 47 deg 02 min 35.0 sec

 

Hah! Looks like we both saw the problem at about the same time. :) You dropped the .04 seconds from the conversion, is this normal?

Edited by Cyclometh
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To convert to seconds with anything more than one decimal place is false precision. Your GPSr's 3-decimal-place minutes is equivalent to a positional precision of about +/- 3 feet.

 

If you kept the seconds to 2 decimal places, you would in essence be claiming precision of +/- 0.005 seconds, which is equivalent to claiming a precision of about 1/2 foot, instead of the 3-foot precision you actually have.

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I concur with the previous comments regarding accuracy. Don't claim more than a tenth of a second, no matter how you arrive at it. Personally, I enjoy decimal degres, but I don't claim the three digits after the decimal when I do the coversion to ddmmss.s.

 

HOWEVER, there IS a valuable use for decimal degrees when searching for an elusive benchmark--provided it has adjusted coordinates. Convert ddmmss.s to decimal degrees. Then set the GPS unit to decimal degrees. W walk around and watch the display. You will notice that it changes with a very small amount of movement. When you're at the exact spot, look down......

 

Often, I've used this method and ended up STANDING ON THE MARK!

 

Practice with a known benchmark with adjusted coordinates. You'll see what I mean!

 

-Paul-

 

PS. I use GPSvisualizer for converting coordinates. If you open two pages at the same time, you can cut and paste the decimal degree version into the maping program. And you can paste them into a word document for use in the field.

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