It's about a tenth of a foot horizontally. Enough to notice when you're checking a HARN derived project to OPUS.
Slightly less than two here, although it's not the horizontal that's the main issue, it's the vertical that really doesn't work. Using the latest ellipsoid heights and the latest geoid really won't match, most of the local HARN points are also first order bench marks so all kinds of info were tied to them, flood plain mapping, sewer, water, road, building projects. You just can't put the toothpaste back in the tube at this point. Shifting it, well...... it's something I can do but how do you make everyone do it?
NAD 83 and WGS 84
No Bueno.
OK - I've got to ask
not true Dave. over the 9500 acre site I am working on the difference is over 1.5 feet. The combined factor for the closest monument is 1.00005980 which makes about 0.12 per 2000 ft. This may not be enough to bother with on most surveys but over 9500 acres, it makes a difference
I agree that it is simple and rigorous (mathematically) to convert between the two but I see no provision in the law that would allow me to provide local coordinates based on my system.I believe I could show state plane coordinates and a mapping conversion angle but that seems rather convoluted. I am tempted to show my LDP metadata, false northing/easting and label a few coordinates.
NAD 83 and WGS 84
There's more to it than that, though.
WGS84 vs NAD83 as a realization equates to about a meter horizontal difference. You can see the difference in OPUS results that publish ITRFxx and NAD83xx coordinates for the same point.
As an ellipsoid, the two are very similar. Similar enough to be interchangeable for surveying applications. So in a lot of survey software, you'll see WGS84 specified, enter a coordinate from NAD83, solve a point some distance from that entered coordinate, and still be in good shape. Or inverse between two positions in lat/long WGS84 and the same two points in NAD83 lat/long and get very similar results (imperceptible differences over 'survey' distances).
The WGS84/NAD83 thing is... nuanced.
2
Well, you could have surveyed the entire thing in state plane. Bearings and distances. Then drawn the map in grid, except for annotation. Then scale the line work up by the CSF. Then annotate the map and write the description from it. No mental heavy lifting there to satisfy the state, not have to fool with the LDP, and get the grid bearings and surface distances.
I'm lazier than I look. 🙂
Kris... Kris:
You don't know the power of the projection. Join us... (in my best Darth Vader voice)
besides, we have cookies.
NAD 83 and WGS 84
Shawn is dead on. The issue that I've seen far too often (hundreds if not thousands of times) are those were surveyors (and others) incorrectly label coordinates as WGS 84 because as Shawn points out that's what the software says when in fact they really have some realization of NAD 83. The next user then tries to perform an unnecessary transformation and things just get ugly from there on out. This can be very confusing.
A case in point - Shawn very correctly points out that you can see the positional difference between NAD 83 (2011) and WGS 84 (G1674) on an OPUS output, yet the OPUS datasheet never says anything about WGS 84. You have to know that the National Geospatial-Intelligence Agency (NGA) has stated that WGS 84 (1674) can be considered virtually identical to ITRF08. Since OPUS provides IGS08 not ITRF08 so you also have know that ITRF08 and IGS08 are virtually identical at the couple of mm level.
Ahh Yes Kris, that sort of thing is actually much more problematic than you think. for instance, once you have scaled your drawing (at some random point), you now have a drawing that looks like grid but its not..ten years from now, who will remember what drawing is grid and which one is sorta grid..who will remember what point you used to scale by? Now you go and draw a new building in your "pseudogrid file" and then how do you stake that with GPS? You have to scale by the inverse of the combined factor, oh but what point did I scale from again?..Once I have a project LDP setup, (which is very easy btw) then I no longer need to worry about grid vs ground. All that converting, file manipulations and keeping multiple files goes away...come on over to the dark side as Shawn said, you will like it!
> Ahh Yes Kris, that sort of thing is actually much more problematic than you think. for instance, once you have scaled your drawing (at some random point), you now have a drawing that looks like grid but its not..ten years from now, who will remember what drawing is grid and which one is sorta grid..who will remember what point you used to scale by? Now you go and draw a new building in your "pseudogrid file" and then how do you stake that with GPS? You have to scale by the inverse of the combined factor, oh but what point did I scale from again?..Once I have a project LDP setup, (which is very easy btw) then I no longer need to worry about grid vs ground. All that converting, file manipulations and keeping multiple files goes away...come on over to the dark side as Shawn said, you will like it!
Hold the phone. Maybe I didn't clear it up well. Once you scale the line work at a traverse point or Southwest corner, it doesn't move all that much, and you didn't molest the coordinates at all, allowing one to label actual state plane values and have all of the power of knowing what the real distance is.
As far as the "in house meta data" goes, WHEN I do this (I rarely do as I leave everything on the grid), I type a note outside the border of the map so when I "zoom extents" then I see that note. Also one goes in the file. It's analog and digital. 🙂
As far as doing the building, first I don't stake buildings with GPS. 🙂 Second, you haven't screwed with the coordinates and it's a building, so stake it where it calls out at the surface or whatever dimensions. All you need is one point and the line and voila, you're on the side of the building.
My point is, all of these hard problems CAN be overcome and CAN produce the best of both worlds without having to invent new physics to make it happen. However, I have no beef with LDP's, especially in the case of Shawn. But to make one for one project, well like I said, I'm lazier than I look. 🙂
OK - I've got to ask
Dave, I like things to be simple
The bunch that uses SPC with a scale factor computed from TS direct shots.
and
The bunch that uses UTM with a scale factor computed from TS direct shots.
I've been using WGS84 since I found that my untouched GPS data computes to within 0.02± of my TS direct shots.
I've had three projects in 10yrs that I've had to report data and all three were different forms. When I have to that I hire a retired surveyor that has kept his Trimble RTK gear on a daily basis.
The difference between what I use and an opus solution is generally on a bearing of N62°±E at 6.3±ft when downloading info from an opus site about 50mi away
0.02
Yes, what you say can be done and many people do that, however the LDP has many advantages over what you propose.
My coordinates/linework match.
I can draw and stake anything at the same coordinates
I dont have to scale by some random point that no one else knows.
I can convert between the LDP projection and any other projection easily.
I can mix conventional and GPS measurements without any adjustment.
an LDP is a great solution for many projects.
"A great many surveyors using GPS - I think more than half - simply do not understand geodesy on even the most basic level."
I certainly would have to agree with that statement, but I believe it is far more than half. And its not just surveyors, it's many of those who request them for some reason they don't even understand...
