> Absolutely, a great question. One that derives from a requirement of the DOT here, and the need/want of GIS folks to be able to incorporate projects into a larger framework. Engineer might have some benefit of being able to use aerial maps, and other GIS data, etc. during the design effort.
The other question that I think is still remaining to be answered is: "How easy is it to set up the design software (Civil 3D, was it?) to allow one scale factor to be used in designing and dimensioning a drawing so that the coordinate base is kept in SPC?" I can do this fairly easily with the obsolete drafting software I use, so I assume that the hotshots who sell state-of-the-art design software have solved the problem, too.
That is, designer enters a dimension for a line, software applies scale factor to entered dimension before calculating coordinates of endpoints of line. When annotation of line length is performed, the reverse operation occurs to annotate the line with the length originally entered.
Yes, if that's how your data collection works then apply the scale factor.
But I would test that. Be sure you aren't doing it twice. Most collectors I've used in the last twenty years will take the raw data and convert it without applying a scale factor.
It's easy enough to check, just write down the raw numbers on a fairly long shot, then hand reduce the coordinates and compare the coordinates to the ones in the data collector.
If you are going to hold state plane then do everything in state plane.
OK, now I see where you're coming from. Thanks for all the help.
Loyal
Think we are saying the same thing.
The way to "get" the LDP surface to the height you wish is a scale factor, yours is 1.000333 the last one I did is 1.000192, but there needs to be an adjustment, otherwise your system will be down at (well near) sea level, and no I've never considered "adjusting" the semi major axis numbers
And from there you have the same issues that a state plane system has, as you move up or down in elevation your ground to grid "accuracy" starts to slide.
But in essence are we not talking about applying a scale to our LDP coordinates just like we do in state plane to get surface coordinates.
There was a time when I thought SPC's were it. But after a few years I got tired of the confusion and......... conversion crap.
Now I use grid bearings and convert everything to ground from the center point and life is better.
John Harmon
I have been campaigning for SPC base on construction projects since the early 80's.
Was too radical for anyone back then. Glad to see everyone has come around to my way of thinking. I'm retired now.
Structure dimensions are basically unchanged. (diff in coord is next to nothing)
Nobody really cares if a hard structure is out of position with the rest of the world by a few tenths. Structure dimensions are more critical if prefab components are an issue.
Everything is rayed out from SPC Control Points that are only maybe 500' apart.
They keep overall, big picture "mis-location" under control.
Who cares if a Vertical Curve length is "off" by a tenth or two vs ground.
SPCs keep the world related.
Especially if big elevation differences are involved.
(i.e. 0.01' per 1000' horiz per 200' elev difference)
Topo? Grid/Ground? We're locating hardscape to nearest 0.1' at best and trees/etc guesstimating nearest foot.
Everything is built to match (i.e. warped in) anyway.
Mighty
No...you're missing an important point.
When the projection is defined, instead of using a scale factor (k) of say .9999 (as is common in SPC Projections, or .9996 as in UTM projections), you use 1.000xxx!
That way your developed surface is ABOVE the NAD83 Ellipsoid, NOT below it at the central meridian (or parallel on a single parallel Lambert).
So as long as your project stays within the design limts, your "elevation factor" can be ignored in most cases.
Loyal
I just want to say thanks to all of you who posted on this topic. This forum is truly an amazing resource.
The last time I used some Txdot spc (ha ha)coords., the control points had coords. on them that were about 90 feet off.
I asked the engineer if he knew they were bad and his reply was, "All cordinates we use here (this county)are considered relative coordinates so it does not matter. A private contractor furnished them and others have told me they were bad also".
Who knows whether they are ground or grid.
> To keep LDP projection grid north identical with SPC grid north one would need to choose the same principle meridian -
Yep, That's the same way I do it.
James
Sounds like they scaled the coordinates from 0,0. In the example I used above, you'll notice a difference of about 50 feet in the northing. With more significant scale factors, the differences would be even larger.
Before you start I would strongly suggest you download and read the articles posted here by Richard Sincovec. If you copy this information to the engineer hopefully you can start a discussion leading to working something out.
All of the Northing and Easting values used inside of Civil3d are assumed to be local. Your engineer can receive Grid data and if you supply the necessary information then he can set up the software to translate the incoming points to localized site coordinates. If you also supply 2D CAD data then he can scale it using basic ACad commands to match, but 3D CAD data is more of a problem because the program will scale the Z values.
The software can also export Grid point data but not alignments, profiles, surfaces etc. You're better off getting the design information in local coords, then convert back to Grid if necessary for layout.
I have read the article "Working with grid coordinates" by the late Richard Sincovec, may he rest in peace. And , had supplied this very same article to the very same engineer last summer when I had begun sorting out these issues.
I am pleased to see the additional articles available and will eagerly read them, as the first mentioned article was excellent.
Thanks, Steve.
Pretty cool, but I thought the 'surveyorsor' was extinct. :whistle:
"Be sure not to truncate the coordinates, it makes it too difficult to "get back" to the state plane value. Also it is much easier to sample in Quads, photos and such into a cad drawing and then "scale up" the quad or photo by the scale factor around 0,0 to get on your coordinate system."
MightyMoe,
While this may sound great on paper, when you convert a SPC from grid to ground it ceases to be a SPC. I can no longer count the number of large design projects were I have been brought on after as the clean up surveyor after project was started and been given coordinates that look like state SPC but were in fact on an undocumented local ground system that the previous surveyor had set up come up with. Just to let you know, the meta data rarely makes it along with the drawings. The problems that ensue are enough to drive me crazy. This is case where it looks like a dog and smells like a dog but is in fact a cat. This goes double for those whom feel the need to scale about a point within the project area. At least when I set up my RTK base on a SPC point scaled about 0,0 my receiver tells me that the autonomous coordinate is significantly different than the given. It is best to make it look and smell like a cat.
If you need to insert background information, it is just as easy to scale and translate the attachment/insert as it it to scale it.
Just to let you know, the meta data rarely makes it along with the drawings
The Metadata is on the drawings-on each sheet in fact.
I can't recall a project that I've ever done where there are coordinates given out without explaining what they are. This has been procedure since I've been surveying in the 1970's and I would never assume anything about some coordinates that are given to me without some metadata explaining what they are based on.
I'm not saying I even like the system of multiplying coordinates up to surface, but that is how DOT's I work with do it. And I've been doing those conversions since 1978 with the first job I worked on for them and never had any problems.
It works fine and I've never even heard of anyone else having a problem except one particular engineer who use international feet for his coordinates. That had nothing to do with scale factors.
If you truncate the coordinates you lose the easy conversion to state plane and back. This will cause all kinds of issues using the surface coordinates, regardless if they are "close" to state plane. If you sample in a quad into your drawing you can't easily scale it to surface, same with photos or other data that comes in at state plane. And getting your drawing to another system becomes a time consuming project, needlessly time-consuming.
