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Very basic GPS tutorial
Ok, the whole sheet of paper represents your state.
The star is true north.
The heavy N-S line is the central meridian. At C/M, true north, and grid north are the same.
As you move east, or west, of c/m, with True North, you have to consider either a positive, or negative theta. If you set up your local system on true north, it’s no longer TRUE, if you move E or W. You have to compute the theta difference between the 2 stations.
The little squares of the grid paper represent feet. These feet vary in size. You have international feet, and USA survey feet. Also, you have a “grid to ground” scale factor. That needs another drawing to exhibit, in profile view. Like a flat postage stamp, on a basketball. I’ve not provided it.
Anyway, many can add to this, and super-over-complexify it. But, this basic drawing needs thoroughly memorized, so you can draw it mentally, at any time.
Think of the two lines, TRUE NORTH, and GRID NORTH, at any given location, as the BACKSITE, that a given grid, or rectangular coord system is set up on. (Of course, there can be any variant thereof, but, it should be related to one, or both of these tools).
I hope to see legislation to mandate a reference to one, or both of these in my lifetime.
But, until then, all surveyors have an ongoing need to understand my simple diagram.
Nate
PS
One more statement. Most survey grade GPS can address my diagram, to the tenth of an arc second. IF you know where, and how to find it burried in the box, called a data collector.
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29 Replies
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I don’t think the flat earthers would agree with this post.
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I used to be a flat earther… (You know, the kind that does not worry about anything beyond basic cogo)
GPS seems to like to make us consider a bigger world, than our little flat spot!
N
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I used to be a flat earther… (You know, the kind that does not worry about anything beyond basic cogo)
GPS seems to like to make us consider a bigger world, than our little flat spot!
N
I dunno Nate.
The earth got “round” (in the survey measurement sense) LONG before GPS came along.
At least it did for those of us in the Intermountain West, when the HP-3800 came along about 1971. All of a sudden even the most casual surveyor had to get his/her head around the effect of curvature, refraction, AND the varying “elevations” of the observations.
Loyal
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Loyal,
Don’t forget Eratosthenes, the first geographer and an early surveyor who figured out the curvature part circa. 240 BC.
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Just set a 12″ spike, put your base over it and hit “here.” Easy-peasy
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Right on Gene.
James R. Smith (Introduction to Geodesy, 1997) talks about this, and names;
Pythagoras c.580-500 B.C.
Aristotle c.384-322 B.C.
Archimedes ~ 100 years after Aristotle
Eratosthenes 276-195 B.C.
Poseidonius ~100 years after Eratosthenes
Just for starters.
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I used to be a flat earther… (You know, the kind that does not worry about anything beyond basic cogo)
GPS seems to like to make us consider a bigger world, than our little flat spot!
N
I dunno Nate.
The earth got “round” (in the survey measurement sense) LONG before GPS came along.
At least it did for those of us in the Intermountain West, when the HP-3800 came along about 1971. All of a sudden even the most casual surveyor had to get his/her head around the effect of curvature, refraction, AND the varying “elevations” of the observations.
Loyal
Of course for SOME surveyors, it’s been round for a long time. (Even the early GLO surveyors were aware.
But, somewhere along the way, a whole batch of surveyors quit worrying about it.
Then, the “big chief surveyor” in town got GPS. And he still doesn’t know what his “big new ‘spensive” survey ‘quipment is doing. His plat says “bearings based on GPS”.
Ok…. Whatever! I’d say there is still a too much “GPS abuse”.
It’s wrong to be doing it for 10 yrs, and still not know…
N
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I used to be a flat earther… (You know, the kind that does not worry about anything beyond basic cogo)
GPS seems to like to make us consider a bigger world, than our little flat spot!
N
I dunno Nate.
The earth got “round” (in the survey measurement sense) LONG before GPS came along.
At least it did for those of us in the Intermountain West, when the HP-3800 came along about 1971. All of a sudden even the most casual surveyor had to get his/her head around the effect of curvature, refraction, AND the varying “elevations” of the observations.
Loyal
Of course for SOME surveyors, it’s been round for a long time. (Even the early GLO surveyors were aware.
But, somewhere along the way, a whole batch of surveyors quit worrying about it.
Then, the “big chief surveyor” in town got GPS. And he still doesn’t know what his “big new ‘spensive” survey ‘quipment is doing. His plat says “bearings based on GPS”.
Ok…. Whatever! I’d say there is still a too much “GPS abuse”.
It’s wrong to be doing it for 10 yrs, and still not know…
N
I agree with you 100%.
It would be funny, if it were not so sad.
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I saw a note Benchmark based on GPS datum.
I was disappointed that it was not an ellipsoid height. It looked like NAVD.
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I saw a note Benchmark based on GPS datum.
I was disappointed that it was not an ellipsoid height. It looked like NAVD.
That sort of lack of knowledge, only will get worser, and worser.
If all surveyors, that use GPS, understood “mapping angle”, it would solve a whole host of problems. Also, heights. And, I’m not fluent with heights…
Nate
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I hope everyone has a great day; I know I will! -
This week’s survey:
From a colleague’s 2013 survey based on the state coordinate system and scaled to surface: Record N0d42’11″W, 5184.49′, our Measured N0d42’10″W, 5184.46′
and Record S89d29’30″W, 2671.01′, Measured S89d29’33″W, 2671.00′
slightly different scale factors, I didn’t try and adjust for that.
I think I’m lucky to be following lots of old timers that understand the math, it seems some of the younger ones are more of the button pusher types. Maybe sitting down and doing calculations long hand back in the day was helpful when the new tech came out.
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This week. Retrace an old subdivision from the ’60’s. For most of it, lots and blocks weren’t followed. But odd meets and bounds. Errors of 10′ are normal. Some places 25′ Retracement is one of the most fundamental activities I seem to perform.
N
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So how about a little quiz.
1. Using NGS data sheets and a calculator or a blank computer spreadsheet, compute the grid azimuth from FY2147 to EZ0604 on the North Carolina SPCS.
2. Using the same resources as in (1) and your computed grid azimuth, compute the geodetic azimuth.
3. Check your answer for the geodetic azimuth using NGS Inverse.
Sorry, no CEUs, but it was engaging, wasn’t it?
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—–The old arc-to-chord correction strikes again because the distance is substantial.
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Looks right to me. It’s good to review this stuff from time to time, so thank you, Nate. It took me the longest time to get this into my head, which had been immersed in the 5000,5000 Flat Earth world for years when I got my GPS rig in ’04 and yes, as others have noted, a number of land surveyors still don’t get it. And the lawyers, don’t get me started….
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“Distance is substantial”. Yes.
I don’t think I’ve ever done one this far apart. I’m a somewhat modern surveyor. DPOS. OPUS, go see “how far off” that old NGS marker is….
I used to tie into old NGS markers, with L-1 GPS.
I often used C&G software, (back in the ’80’s.) Which had a great sunshot routine. It would spit out both geodetic, and grid north. Multiple sunshots can improve a large total station traverse.
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It always amazes me to see how much trust people put in a sales person. They purchase the most expensive equipment they’ve ever looked at, only to use and trust the work flow handed them by a salesman.
Don’t get me wrong. There are a handful of people in sales who know thier stuff. I don’t know any who qc the data and come by to stamp the map when it’s ready. We certify products as being done under our responsible charge. That doesn’t mean we do each step. It means we can, and we were involved enough to say it was done correctly.
This basic lesson is good discussion. Now we should put feet on it by figuring out when and how our software uses (or doesn’t use) Theta…
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Nate, your diagram is an excellent educational piece. You should teach.
The quiz does show a fairly long line (11.26 miles), but perhaps not that long for a GPS vector. It also crosses the central meridian for the NC State Plane, so one convergence angle is negative and the other positive. That’s of no consequence unless you think that the two need to be averaged for some reason. The NGS Inverse azimuth is 114d 00m 41.9429s. I got 114d 00m 41.9674s when I computed it from the grid azimuth.
Here are a couple of educational resources that make good reading. The first is Jan Van Sickle’s course at Penn State; lesson 6 is the applicable one: https://www.e-education.psu.edu/geog862/home.html
The second is from an old POB and, under the last section headed Plant North, it describes the importance of the proper sign for the convergence angle: https://www.pobonline.com/articles/88903-geodetic-surveying-made-plain-where-s-north
Lots of opportunities to learn in this day and age.
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Strictly speaking this is not a GPS tutorial, but rather a projection system tutorial.
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