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shunra

NAD83 datum

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I just started benchmarkign this week. I found and identified 16 benchmarks by their descriptions, as the NAD83 datum was no help whatsoever, sometimes the coordinates were as much as 100 yards off. I thought NAD83 (as opposed to NAD27) was supposed to be almost identical to WGS84... How do you professionals deal with that?

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quote:
Originally posted by Abu Shachaf:

I just started benchmarkign this week. I found and identified 16 benchmarks by their descriptions, as the NAD83 datum was no help whatsoever, sometimes the coordinates were as much as 100 yards off.


Those were probably scaled marks. That is someone read the description, looked at the map and came up with coordinates that way. Needless to say, it won't be very accurate. Those are truly called "Benchmarks" - the height is accurate, but not the position. Now Adjusted marks you should be able to walk right up to. Any error found will be within the error of your GPS. These are "triangulation" stations - where the horizontal position is very accurate.

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Out of the last 6 I did,yesterday, 2 I did not measure or make a note of distance off,but 1 was 508' ft. off @177*,1 was 208' ft. @110*,1 was 163' ft.@ 100*,1 was 540' ft. @ 112*.

 

WHEN ALL ELSE FAILS

*GEOTRYAGAIN*

TAKE PRIDE IN AMERICA

http://www.doi.gov/news/front_current.html

1803-2003

"LOUSIANA PURCHASE"

http://www.lapurchase.org

"LEWIS AND CLARK EXPADITION"

http://lewisclark.geog.missouri.edu/index

 

Arkansas Missouri Geocachers Association

www.ARK-MOGeocachers@yahoogroups.com

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It must be remembered that most (nearly all) of the NGS benchmarks we're hunting for were set before GPS had even been imagined. The official coordinates for a benchmark may not be 'accurate' by today's datums or GPSr standards, but they are accurate considering how many of those original coordinates were derived.

 

A short explanation that may help you understand the coordinates can be found in the Benchmark FAQs, particularly under the heading 'Why do the coordinates of some benchmarks seem to be way off?' You'll find that the coordinates just get you close. You almost always have to rely on the written description to find the mark.

 

Professionals deal with the datums with much study, use, sweat and tears.

 

Keep on Caching! (and Benchmarking!)

- Kewaneh

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ABU,

 

GG touched on the problem. The datum is not the problem. There are both horizontal and vertical controls. Most vertical controls have their horizontal coordinates scaled and these may be off by a signifcant amount. Read the top of the data sheet to see the possible error. Horizontal controls are ALWAYS accurate and I have been able to walk right up to them with my GPS. You should look at the original data sheet to see the data. I usually go to teh NGS website and do a search for horizontal controls only because I'm lazy!

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LIke the guys have been saying, the horizontal should be good unless it is strictly a vertical point. If it is strictly a vertical the lat/lon will only e to the nearest second rather than carried out the 4 or 5 pdecimal places. There are times as well where the selecctive availability may be turned off and it will greatly affect your accuracy. The hard part is that we never know when it will happen.

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SA has not been implemented since Clinton ordered it turned off back around 1998.

 

Scaled positions could be off by as much as 6 seconds. The survey crew plotted their marks on a USGS map and then scaled the Lat and Lon from that information. Without this scaled position, you could not do a search by area for marks and that would seem the primary goal of scaling non-horizontal marks.

 

Mike

Survey Tech (Retired)

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quote:
Originally posted by elcamino:

Scaled positions could be off by as much as 6 seconds.


I'm too lazy to reach for my calculator - 6 seconds to us mortals is how far?

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quote:
Originally posted by GeckoGeek:

I'm too lazy to reach for my calculator - 6 seconds to us mortals is how far?


It depends on your position. For example, I figure that:

 

@ Lat=30°

6 seconds Lat is roughly 607 feet

6 seconds Lon is roughly 526 feet

 

@ Lat=40°

6 seconds Lat is roughly 607 feet

6 seconds Lon is roughly 465 feet

 

@ Lat=50°

6 seconds Lat is roughly 607 feet

6 seconds Lon is roughly 390 feet

 

(Please correct me if I'm wrong! icon_smile.gif)

 

Cheers ...

 

~Rich in NEPA~

 

--- You might own the cache, but geocaching.com owns you. ---

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Rich - you are correct on LONG (I think - I didn't run the numbers). It sure does depend on your LAT. But what is really COOL is LAT. 1 minute LAT is ALWAYS EXACTLY 1 nautical mile = 6000 feet. Since we rarely deal with seconds on GC.com, decimal minutes are cool: 0.1 min LAT = 600 ft. 0.01 min LAT = 60 ft. 0.001 min Lat - 6 ft. Always. At the equator or the polls. That's what my navigation text tells me. BTW - a second of LAT = 6000 / 60 = 100 ft. So 6 seconds LAT - 600 ft always.

 

Engineers design it to nanometer tolerances, measure it with a micrometer, mark it with chalk, and cut it with an axe.

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Except that a nautical mile is not 6000 ft, but 6076.131

 

So, to adapt your calculation:

a second is 6076.131 / 60 is 101.269 ft

And 6 seconds are 607.613 ft.

 

Veni Vidi Cachi

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If you want to compute the exact distance of a second of either latitude or longitude go to the NGS web site and select 'geodetic tool kit," then select "Inverse/Forward." Select the Inverse option and enter the latitude/longitude of the area you're interested in as the first point. For the second point simply change the latitude or the longitude (not both at the same time) by 1 second and you will get the value in meters. Be aware that the value will differ slightly depending on which geodetic ellipsoid you select, and there are more than 20 different ellipsoids in use around the world today.

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Should we not then have one System to define the World as a whole?

Isn't that the WGS?

 

WHEN ALL ELSE FAILS

*GEOTRYAGAIN*

TAKE PRIDE IN AMERICA

http://www.doi.gov/news/front_current.html

1803-2003

"LOUSIANA PURCHASE"

http://www.lapurchase.org

"LEWIS AND CLARK EXPADITION"

http://lewisclark.geog.missouri.edu/index

 

Arkansas Missouri Geocachers Association

www.ARK-MOGeocachers@yahoogroups.com

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There is a single global system, but it's not World Geodetic System 1984 (WGS 84), it's called the International Terrestrial Reference Frame (ITRF), and it is a product of the International Earth Rotation Service (IERS) headquartered in Paris -- http://www.iers.org/ . WGS 84 is the coordinate system defined for GPS by the U.S. Defense Department. GPS is just one of several technologies used by the geodetic community to develop national reference datums. Other technologies include: Very Long Baseline Interferometry (VLBI), Satellite and Lunar (SLR/LLR), and Doppler Orbitography and Ranging Integrated on Satellites (DORIS). While GPS is currently the only stand alone positioning service, and the one most people are familiar with, there is also the Russian GLONASS satellite system, similar in design and concept, but no where near as robust. Some of the high-end GPS units can also integrate GLONASS signals to help improve the GPS solution. The GLONASS coordinate system is called Paremetry Zemli 1990 (PZ 90) and is very close in its definition to WGS 84. Over the next several years the European Community is committed to launching a satellite navigation system called Galileo. This system will certainly not use WGS 84.

 

Countries and groups of countries define geodetic datums to fit their national needs. While considerations of global relationships are always part of the equation, there is no global geodetic authority, just international technical working groups and research institutions such as IERS that try and coordinate and facilitate communication and cooperation between countries. For example, the United States and Canada developed the North American Datum of 1983 (NAD 83) beginning in the early 1970s. For a variety of reasons, Mexico and most of the Caribbean, Central and South American countries could not participate in this effort. Today, after decades of technological and political developments, most of the South and Central American countries are integrated into a system call SIRGAS. In similar fashion in Europe they have developed the European Reference System (EUREF), while Australia has adopted their own national system Geocentric Datum of Australia 1994 (GDA 94). All of these systems, while different in name have a common link through ITRF.

 

In 2002, the Defense Department released a new flavor of WGS 84 that is consistent with ITRF at the +/- 2-3 cm level. The system is now appropriately called WGS 84 (G1150). The G1150 stands for GPS week 1150 in which the new definition was adopted into the satellite orbit computations.

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As always thanks again DaveD for the link.

I asked in another forum but I guess you did not catch it.

Where you in Washington D.C. on the 9/18/2003?

around the Capitol about Noon.

The reason I ask is I saw 4 men with NOAA Hardhats coming from the Construction area while I was there.

 

WHEN ALL ELSE FAILS

*GEOTRYAGAIN*

TAKE PRIDE IN AMERICA

http://www.doi.gov/news/front_current.html

1803-2003

"LOUSIANA PURCHASE"

http://www.lapurchase.org

"LEWIS AND CLARK EXPADITION"

http://lewisclark.geog.missouri.edu/index

 

Arkansas Missouri Geocachers Association

www.ARK-MOGeocachers@yahoogroups.com

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I knew I should have stopped to say Hi,but with the Search by the Secret Service my mind was still not woking fast enough yet,and for me that was quite the experience!!!!!!!!!!!

The only other time I have been so close to the S.S. was when I got to meet President Bush earlier this year in Pierce City,Mo.

I plan a visit back to view the Original Documents that the President put out for display on the Rotunda the Day before I was there.

And maybe see a few of the other Benchmarks and Historic places.

 

WHEN ALL ELSE FAILS

*GEOTRYAGAIN*

TAKE PRIDE IN AMERICA

http://www.doi.gov/news/front_current.html

1803-2003

"LOUSIANA PURCHASE"

http://www.lapurchase.org

"LEWIS AND CLARK EXPADITION"

http://lewisclark.geog.missouri.edu/index

 

Arkansas Missouri Geocachers Association

www.ARK-MOGeocachers@yahoogroups.com

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Thanks for a very informative thread. I get that scaled means don't trust the coordinates, but the NAD83 vs. WGS84 issue was not directly addressed, right? I have only logged 9, but when I looked back at my photos, I see that the coordinates are pretty much spot on (even for one that is scaled). But let's assume that there could be significant differences between NAD83 and WGS84. I have an eTrex set for WGS84 (for GC'ing). If I'm benchmark hunting, I should set for NAD83 to get the best accuracy, at least for those whose coordinates are not scaled. Anything wrong with this approach?

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Dumples -

 

For all intents and purposes, NAD83 and WGS84 are identical. It wouldn't be necessary to change datums in your GPSr. Similar discussions and explanation can be found in these threads:

 

NAD83 or WGS84

 

What's typical for coordinate accuracy?

 

NAD vs. WGS84 When do you change?

 

Benchmark Map Datum vs Geocache Datum

 

Convert Nad 83 to WGS84??

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The bottom line is that NAD 83 and WGS 84 differ by about 1 m in both horizontal and ellipsoid

height, so for bench mark hunting there's no difference. There is significant difference for surveying

but that's beyond the scope of these discussions. I'm giving a presentation to the North Dakota

Association of Professional Land Surveyors tomorrow where this will be a significant portion of the

the morning's discussion.

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MODOT, Missouri Dept. of Transportation are doing some RTK in the area right now and they stated that they had just been to one of your classes, DaveD.

 

You are a busy fella it seems like.

keep up the good work,maybe one of these days I will get to go to one of your lectures or classes.

we can hope..............................

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