IF you do a survey, and use a HERE, for your GPS base, with ground coords, it SETS the brgs to GEODETIC north, at the base. Now, you perform your survey about a mile EAST of the base, and put on the plat BOB = TRUE north, via GPS. Then, you do another survey 5 miles to the east of the 1st one, and you do the same thing, with the GPS base, on a second base site, and then you decide to TIE the 2 surveys together, are they on the same brg system, or not? They BOTH say that BOB = True North.
Now, same scenario, but you after setting up your base, tell it to do a here, THEN after connecting to your rover, you ask it for the convergency angle, and apply it, then you are ON grid brgs. With BOTH surveys. You state on your plat that you are on GRID brgs.
Now, how much difference is there in brgs, between the 2 surveys? Materially, none.
Nate
Note quite Nate. Every time you do the here position with WGS84 in the back ground, you have to flatten the project to 10k,10k, and when you do that, you essentially make a theata for that project. Now, so long as your project isn't too big, you essentially have geodetic bearings, but not exact, ever, unless you're working with LLH (I don't know anyone who does this).
As far as everything should be on the grid, for that, I agree.
Shhhh! You have discovered how we pincushion ourselves at every turn! Trade Seekrits!
True north?
"In descriptions of boundaries, the use of true has legal significance and, except in rare instances, refers to values based directly on astronomical observations."
Geodetic Glossary - NGS
So what GRID you on? The possibilities are endless. You need to set up a projection. Then you can move the base around and stay on the same Grid. The use of the HERE Key sold a lot of GPS systems but other than that is useless and dangerous. For the most part using the HERE Key to establish a local projection shows the user doesn't understand what he is using. It probably goes downhill from there.
:good: :good:
NAD 83. That uses the middle of Arkansas, for the C/M.
N
Ahhh Nate...
NAD83 is a DATUM not Projection per se. There can be (and are) a number of “NAD83 Projections” (and realizations thereof), that one might use in Arkansas.
I RETRACE surveys that were (and are) defined by True Bearings, NOT State Plane or UTM Beraings.
Loyal
Ahhh Nate...
And that's why I listen to Loyal.
:good: :good:
Ahhh Nate...
Right on Loyal.
I understand what you are saying Nate, and quite frankly I agree.
James
IF you do a survey, and use a HERE, for your GPS base, with ground coords, it SETS the brgs to GEODETIC north, at the base. Now, you perform your survey about a mile EAST of the base, and put on the plat BOB = TRUE north, via GPS.
I think I know what you're doing. You are actually on grid bearings if you do what I think you're doing with the HERE button.
The central meridian passes through your base point and only along that longitude is your survey true north.
on the plat I would say that Bearings are Grid based on a Transverse Mercator Projection; origin lat.=, origin long.=: or something similar.
I don't think the bearings are true north unless your data collector does something different than mine.
He is not talking about being on just any Grid North, he is talking about being on State Plane Coordinate Grid North. The coordinates are not State Plane. The distances are not grid, they are ground. But the Bearings are based on SPC Grid North.
James
Yes, I figured he was talking about rotating to state plane somehow. But the HERE point also creates a grid.
There are many grid systems: state plane is only one.
Grid brgs, ground dist. Correct.
Now, we do have geodetic north, at the base. but, because it is REALLY a rectangular coord system, or grid coord system, BBUT it is based on geodetic at the base. INSTEAD, use the CM for Arkansas, for your "true" north system, and your brgs will be ARKANSAS SPC BRGS, and ground distances.
Nate
There is also convergence to worry about. That can cause a divergence between bearings on ground systems vs. grid systems, depending on how far you go horizontally.
Grid brgs, ground dist. Correct.
To get on ground distances there will need to be a scale factor applied. I know that using my data collector with a HERE and an assumed coordinate that I've just created a grid system on the ellipsoid and will be inversing grid distances.
Are you talking about one of those bastardized systems, often called something like "Modified State Plane"...?
I hate those systems. As projects have gotten larger and larger, those systems have been creating more and more problems. I'd much rather use a grid system like State Plane or UTM, and then display a ground distance if necessary. Or another solution that works really well in many circumstances is to create a "Low Distortion Projection", and use that. Both are preferable to the "Modified State Plane" systems, where a CSF is used to multiply coordinate values, and come up with "ground" coordinates that are different from the grid coordinates.
