Author Topic: Question for Lyle (and the group)  (Read 7702 times)

Barry G

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Question for Lyle (and the group)
« on: December 14, 2012, 05:14:05 PM »
Lyle, when you're over here in the US working with groups do you set you sat/nav to UTM/UPS and if you do, do you also set to NAD 27 and Units = yards or meters? Thanks is advance!

Barry
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captain paranoia

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Re: Question for Lyle (and the group)
« Reply #1 on: December 14, 2012, 06:03:24 PM »
The datum you select should match the datum used for the whatever map you're using.

Basically the datum allows the GPS receiver to convert between WGS84 coordinates computed by the GPS engine from the postion data sent by the satellites (which use WGS84 coordinates), and coordinates under the mapping datum; think of it as a 'currency converter', only the values are position, rather than monetary value.

This website might help:

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

It suggests that most USGS topgraphic maps are based on the NAD27 datum, so select NAD27 on the GPS unit; that way, the coordinates it present should place you correctly on the map, without any strange offsets (such as the NAD83 datum offset highlighted in blue on the above website page).

The units of distance is your choice; whichever you're more familiar with.

Barry G

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Re: Question for Lyle (and the group)
« Reply #2 on: December 14, 2012, 06:54:33 PM »
Thanks a bunch Captain!
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Lyle Brotherton

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Re: Question for Lyle (and the group)
« Reply #3 on: December 15, 2012, 08:06:43 AM »
Hi Barry

In practical terms this is exactly what you need to do:

Goto your fifth screen headed MENU
Scroll down to SETUP
Select SETUP
Scroll down to UNITS
Select UNITS
Scroll down to MAP DATUM
There are 100 Datum to choose from, 11 of which are based on the NAD 27 (North American Datum)
Select NAD 27 CONU
This will read NAD CONUS when you have selected it and is the map datum for the Continental United States

Now select POSITION FORMAT
There are 30 formats to choose from
Select UTM/UPS (The Universal Transverse Mercator/Universal Polar Stereographic coordinate systems)

Now select UNITS
You have 4 choices
Select STATUTE

Now select NORTH REF
You have 4 choices
Select GRID (because you are navigating only with your map)

The VARIANCE (Grid Magnetic Angle) This will default to AUTO and in your area this will be 0 DEGREES

Now select ANGLE
You have 2 choices
Select DEGREES

In SYSTEM
You will have 3 choices
Select WAAS (Wide Area Augmentation System) in your area this will improve your units GPS accuracy, integrity, and availability.

Whilst on the SYSTEM screen check your software ersion, this should be (at time of writing) 3.40
« Last Edit: December 16, 2012, 08:59:40 AM by Lyle Brotherton »
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Barry G

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Re: Question for Lyle (and the group)
« Reply #4 on: December 15, 2012, 01:39:06 PM »
Thanks Captain and Lyle. Lyle that's just what I needed in directions, I tried to follow your video on this but in the video I could not see the screen very well and I was not sure. I love this stuff!!!

Barry
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Barry G

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Re: Question for Lyle (and the group)
« Reply #5 on: December 16, 2012, 05:09:10 PM »
While practicing with my Etrex and the new settings Lyle gave me I tried something and here's the results.

1. With the sat/nav set to Nad 27 conus I took a waypoint of my home in the driveway:
    RESULTS = e0601547
                      n4725445
2. With the sat/nav set to Nad 83 (datum on the USGS map) taken in the same spot in my driveway:
    RESULTS  = e0601577
                       n4725670
3. Using a USGS map and a corner Roamer scale I read = e0601560
                                                                                       n4725647

My Conclusions:

      Follow the advice that when using a sat/nav set it to the datum on the map because you could be 25% off as in this case, and 25% of 1,ooo sq. meters is as you know 250 meters which is a good amount in say a wooded area or passing a person laying down unconsious.

My math could be off a bit, but you guys get my point! I love this stuff!

Barry
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Brian

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Re: Question for Lyle (and the group)
« Reply #6 on: December 16, 2012, 06:08:10 PM »
While practicing with my Etrex and the new settings Lyle gave me I tried something and here's the results.

1. With the sat/nav set to Nad 27 conus I took a waypoint of my home in the driveway:
    RESULTS = e0601547
                      n4725445
2. With the sat/nav set to Nad 83 (datum on the USGS map) taken in the same spot in my driveway:
    RESULTS  = e0601577
                       n4725670
3. Using a USGS map and a corner Roamer scale I read = e0601560
                                                                                       n4725647

My Conclusions:

      Follow the advice that when using a sat/nav set it to the datum on the map because you could be 25% off as in this case, and 25% of 1,ooo sq. meters is as you know 250 meters which is a good amount in say a wooded area or passing a person laying down unconsious.

My math could be off a bit, but you guys get my point! I love this stuff!

Barry


Good for you for experimenting.  Very good for you.  I have two additional thoughts.

First, IMO there are two kinds of "knowing" — intellectual and emotional.  In a previous life, I sometimes taught how to diagnose some things.  My students dutifully memorized what I told them to and answered test questions correctly.  Yawn.

But there came a time when they actually had to commit themselves to what they learned in a real-life environment, and when they did, and they found what they'd learned actually worked, they were impacted emotionally in a way which cemented their knowledge.  (I know because up to three years after I'd taught them, some would return to tell me, with wide eyes and surpassing enthusiasm:  "Remember what you taught us about ...?  Well, it works!")

Having compared coordinates for the same location using different datums, I suspect you'll never forget the importance of using the correct datum.  And that's a big deal, at least in these parts.

Second, in teaching GPS/SATNAV to our SAR members, we strongly emphasize the importance of using the correct datum.  We have a powerpoint slide of one location correctly placed on a map with the proper datum, but incorrectly placed using 5 - 6 incorrect datums. 

Another slide shows what happens if you go to the right coordinate numbers, but you are using the wrong datum:  you're on the wrong side of a river in a deep valley with no way to communicate with the outside world and with the only river crossing 2 miles away along a gnarly trail.

In fact, every time we have a discrepancy in location (my receiver doesn't agree with your receiver) the first thing we check is whether or not we are using the same datum.

FWIW.  We (our county and the other 8 CORSAR counties) used to use NAD27, because it was the datum used to create all our 1:24,000 USGS paper topo maps. 

But we changed a year or two ago to WGS84 because we no longer use topos printed by the USGS: we now print our own maps using National Geographic TOPO! or TNP, and can select which datum we wish to use.  (Air and Marine assets use WGS84, and it's just easier for us to do so too.)

Barry G

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Re: Question for Lyle (and the group)
« Reply #7 on: December 16, 2012, 09:20:31 PM »
Brian, thanks for the encouragement! All of this is new and exciting to me the sat/nav, using and learning about datums, something I never even heard of actually and UTM. I had heard of UTM most of my life but never delved into it. I actually like UTM better than Longitude and Latitude, but maybe it's just that's it's new to me or do you guys think UTM is better or worse than Lat./Long. for navigating, generally?

Barry

P.S. thanks again Lyle for the help and Etrex setup!
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Lyle Brotherton

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Re: Question for Lyle (and the group)
« Reply #8 on: December 16, 2012, 09:34:08 PM »
Logically Barry UTM is easier to work with, because it is based on a metric system. Having said this, familiarity has as much to do with ease of use as anything. Some of the aircrew I train work all day with lat/long and find it just as easy as UTM and I know personally that when I have been using lat/long for extended periods it seems just as easy.
“Opinion is the medium between knowledge and ignorance” - Plato

captain paranoia

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Re: Question for Lyle (and the group)
« Reply #9 on: December 17, 2012, 12:59:25 PM »
Barry,

You have measured a difference of 17m E and 23m N between map and GPS position, using the NAD83 datum, a distance of about 29m.

Now you need to ask where that error comes from: there are two sources; the GPS position and the mapping accuracy (accuracy of the map creation, and accuracy with which you can read the map).

If the map is 1:25000, then you'll be doing well to read a position to 10m accuracy (0.4mm on the map) with a conventional 10-division Romer; 25m, or 1mm, or one quarter of a Romer division is more reasonable.

Assuming you have WAAS enabled, and can see a WAAS satellite, and there aren't any large buildings or cliffs nearby that could give strong multipath errors, your GPS receiver ought to be giving about 3m accuracy (on average; between 1m and 7m in practice).

So the most likely cause of error is the precision with which you can read the map.  This is generally true with an SBAS-corrected, multipath-free GPS receiver and a 'walking map'.

Some useful info at the NGS:

http://www.ngs.noaa.gov/faq.shtml

And some useful utilities they provide, including NADCON to convert between NAD27 and NAD83 coordinates:

http://www.ngs.noaa.gov/PC_PROD/pc_prod.shtml

Hmmm... Maybe I need to actually play with NADCON, as closer reading of the description makes it sound likes it's intended for batch operation, rather than entering coordinates via a GUI...
« Last Edit: December 17, 2012, 05:40:36 PM by captain paranoia »

Barry G

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Re: Question for Lyle (and the group)
« Reply #10 on: December 17, 2012, 06:04:50 PM »
Thanks Captain for the info and the time you devoted to assemble and send it. I'll need some time to digest it and fully understand it, but I've got most of it. When I did the sat/nav stuff there were no obstructions what so ever and at the start I assumed the map readings would be the less accurate of all the readings, but the USGS topo is the map of choice and I figured I should use the datum  on the sat/nav closes to the map coordinates and that was NAD 83. Oh, I did leave out one set of readings and that was WGS83 which was only several meters different from NAD 83 numbers. I love this stuff, it's already made me several years younger (now if I can only remember how old I am).

Barry
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captain paranoia

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Re: Question for Lyle (and the group)
« Reply #11 on: December 17, 2012, 06:29:21 PM »
I was going to post a comment about UTM, and how it's benefits were true angles and almost true distances.  But I thought I'd better check on the different projections that can be used, and found this:

http://egsc.usgs.gov/isb/pubs/MapProjections/projections.html

And now my brain hurts...

I'm still inclined to suggest that UTM allows us to read bearing and distance off a map directly, or to calculate distances from UTM coordinates using a simple Pythagorean triangle, rather than the more complicated trigonometry required if using lat/long.

I read on the USGS site that they abandoned the Polyconic projection in the mid 1950's, and the 1:24000 series maps use a Transverse Mercator projection of some sort... Yet more brain pain:

http://en.wikipedia.org/wiki/Transverse_Mercator_projection

I'm off to lie down in the dark for a while...

captain paranoia

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Re: Question for Lyle (and the group)
« Reply #12 on: December 17, 2012, 06:44:23 PM »
> When I did the sat/nav stuff there were no obstructions what so ever

It's reflections off large buildings, or large rock formations you need to worry about.  But then, if there are things large enough to obstruct, they'll probably reflect, too...

> I assumed the map readings would be the less accurate of all the readings

That's correct.

> but the USGS topo is the map of choice

I've no direct experience of USGS maps, but they have an excellent reputation.  But there's a limit to to the accuracy you can print at that scale, or read position with any great confidence; after all, the map needs to use lines that you can actually see on the map, so they have to be a minimum width (say 0.2mm?), but that line width represents 5m on the ground.  This printing resolution issue is further complicated by the use of cartographic symbols for certain features, which mean they are printed much larger than they really are; roads, for instance.

> I figured I should use the datum on the sat/nav closest to the map coordinates and that was NAD 83.

Correct again.

 ;)

Barry G

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Re: Question for Lyle (and the group)
« Reply #13 on: December 18, 2012, 12:06:34 AM »
Hey Kevin, when you enter the dark room to lay down be quiet because many of us have been in this room for a longgggggg time. I've just spent the last 3 weeks of my life reviewing the issue of lat/long .v UTM. All of the lititure suggests one use the system that best suits their needs/wants regarding navigation. I have come to a personal conclusion that UTM system is generally best for foot (placing one in front of the other) navigation. Why, because it can direct us to within one meter (hypothetically) of a known piece of earth but in reality ten meters is also a small piece as is lets say 50 or 100 meters. So, what are your needs? The UTM system is less complitated and is the system that creates the earth flaten out with the least amout of distortion and you never have to deal with more than a 1,000 meter square of earth at a time. Most people can relate to what 1 meter, 10 meters and even 50 meters looks like in the real world, rather than try to figure what 6, 7, 8 minutes looks like in the real world. Again, it's what you need and want from a map and system. I think if you're a map and compass person UTM goes hand in hand with that. Now rember Kevin, this coming from a guys who's been in the dark for most of his life!!!!!!!!!! I love this stuff!

Barry
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Callum

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Re: Question for Lyle (and the group)
« Reply #14 on: December 18, 2012, 02:02:57 PM »
Barry, something you might want to try: check the same spot daily and record the grid references given to see how accurate you satnav is. Remember to give the satnav time to settle plus have a clear view of the sky. It would be interesting to see your findings.