Grid 2 Ground

The textbook method is to use state plane coordinates and use the average combined factor for each line length (the average for the ends of the line you are getting a distance for) and rotating the bearings by the convergence angle. However, if all of your scale factors are 0.9999xxx (four nines), that's so close to 1, that I would think an average combined scale factor is good enough. Or picking one that is around the "Median" elevation of the project.

Around here, all of the combined scale factors are always 0.999xxxx (three nines). And any given project can go from the foothills to the mountains with bigger elevation changes than normal. Only then would I worry about it. that's my opinion, anyway.

(You are talking about State-Plane grid to another grid at closer-to-ground elevaton. They're both on cartesian-coordinates grids)

Seriously

Your scale factor differences are in the range of 2 / 100,000'ths. Are you seriously going to try to convince me on (what I am assuming to be) a rural survey that you are going to measure that tight? First off, the OPUS solutions aren't that tight. Then if you want to traverse between points you'd have to use a lot better procedures than most surveyors use - but maybe you do.

I don't think you need to make any corrections for any thing.

The scale factor differences applied over a mile would only make about 0.10' of difference - if that much.

I don't know what version of Carlson you have but on the more recent versions SurvNET can produce two coordinate files, one at grid and one at ground. You need to adjust your traverse at grid (State Plane) and then check the boxes shown below.

Hope this helps.

Now . . . how are you going to clean your screen?

> I have 12 OPUS-RS points spread over about 800 acres - 6000'± N-S x 6000'± E-W, with a VERY abrupt 500'± change in elevation from W to E. The points are about evenly divided between the upper level, if you will, and the lower. Per OPUS, I have combined factors ranging from 0.99993969 to 0.99996187.

> So, given the above info, and given that I can't be persuaded to reduce my terrestrial measurements to grid, what would y'all recommend as a means of converting these grid values to ground?

Well, the practice that I follow in a situation like that is to use one average Combined Scale Factor for the whole tract to compute surface distances for map and metes and bounds description. In your case, the average CSF = 0.999951 is only about 10ppm off the high and low values you quote. That's an error of 0.05 ft. per mile in absolute terms, which seems fairly trivial for practical purposes.

Naturally, on the map and in the metes and bounds description you mention the value of the average CSF that was used in computing the surface distances so that someone so inclined can convert them to grid distances for exact computations.

For what it's worth, here's an example of the note I use:

>Distances are Horizontal Surface Distances in units of US Survey Feet, computed using a project average Combined Scale Factor of 0.999909
>Surface Distance = Grid Distance / 0.999909.

Seriously

I don't think you need to make any corrections for any thing.

Dave's right. I'm doing a project right now that perfectly illustrates this. I'm using a local coordinate system based on work I'd done in my project area several years before and expanding it out to cover a new project. The scale factor I'm using is 1.0, ground distances, geodetic bearings. I had to tie in 2 DOT control points relating to some very tight work they did using basterdized SP grid coordinates. The difference I derived between my measured ground and their record grid in 6807' was 0.12' with a change in elevation of more than 500'. So unless your doing a project that extends for many miles and your aiming for first order work ...

No, really

I certainly wouldn't presume to try to convince you of anything. Please, bear in mind that I'm a nitwit just embarking on a voyage into the world of GPS.

As such, try to think of me as a humble supplicant seeking enlightenment from such exalted eminences as yourself, in order that I not bring shame on the family.

God bless you for your kindly assistance. 😉

SS

Thanks again! That's the answer I was looking for, which validates the procedure I actually used. And, you were nice about it.

SS

😀 😀

> Thanks again! That's the answer I was looking for, which validates the procedure I actually used.

You're welcome. On even larger projects where there is more variation in values of Combined Scale Factor, the approach I've used is to plot representative values across the mapping area, and take an average of those sampled values. Star*Net will automatically compute an average value, but if you have lots of control points and other positioned points in one part of the project and others are somewhat sparse, the average will be somewhat skewed in favor of the value of CSF around the part of the project with the most points in the adjustment. The sampling approach avoids that.

No, really

To me a nitwit is someone who never asks questions because they presume to already know all the answers. You sir, are anything but. Go forth and bring great honor on your familly.

I hope I wasn't rude.

But I agree with Kent. The scale factor might not significantly improve the distances, but you might as well apply one, since it's easy to do with today's softwares. Like reading a distance on an edm, and rounding to the nearest tenth, because you don't need it to the precision is reading....why? Might as well go the slightly greater step so as to no longer pollute the data.

No Tom

you weren't rude- your response was helpful.

It just irritates me when people confuse their more extensive knowledge of a subject with intellectual superiority and seek to demonstrate that imagined superiority with disparaging comments.

Thanks,

SS

Sarge

I really, and I mean really, hate molesting perfectly good coordinates. Years from now, that paper that shows what molestation to them occurred will be gone and you'll be left wondering what happened.

So, in the RARE cases where I work on the surface as opposed to the grid, and I abhor scaling, I draw the map, totally and completely in grid, and I mean everything. The VERY last thing you do, is you scale the line work and only the line work up to surface. THEN you annotate the map.

Voila, you've not molested your coordinates, they are still lily white, and you have your surface distances.

FWIW, 4 9's, use a CSF averaged for the tract but don't muck with the coordinates. Just the linework.

Intellectual superiority

> It just irritates me when people confuse their more extensive knowledge of a subject with intellectual superiority and seek to demonstrate that imagined superiority with disparaging comments.
>
> Thanks,
>
> SS

Even if someone were (or thought they were) intellectually superior, there's no reason to be rude.

I used to have a crew chief over me that would have a absolute fit whenever I or someone did something "wrong" or made a mistake. Half the time, right after he had one of his tantrums he would do something extremely stupid. Usually when You demonstrate some kind of 'better-than-anyone-else' attitude, you end up eating it. Open mouth, insert foot.

Kris

Don't you think that what kind of product you put out could depend on who your client is and what it is going to be used for?

Sarge

> I really, and I mean really, hate molesting perfectly good coordinates. Years from now, that paper that shows what molestation to them occurred will be gone and you'll be left wondering what happened.
>
> So, in the RARE cases where I work on the surface as opposed to the grid, and I abhor scaling, I draw the map, totally and completely in grid, and I mean everything. The VERY last thing you do, is you scale the line work and only the line work up to surface. THEN you annotate the map.
>
> Voila, you've not molested your coordinates, they are still lily white, and you have your surface distances.

I've done that for years. Now we use Carlson, and it lets me annotate with grid distances, ground or both in one step. Grid bearings, geodetic or both. Grid area, ground or both. No scaling of line work. Slick!

Carlson oops

> I've done that for years. Now we use Carlson, and it lets me annotate with grid distances, ground or both in one step. Grid bearings, geodetic or both. Grid area, ground or both. No scaling of line work. Slick!

Well, one fly in the ointment: When annotating arcs, the chord bearing is always relative to the acad WCS. So if your drawing is in grid, you can annotate away with line and curve tables and the line table will have geodetic bearings, but the curve table will have grid bearings. Yikes

Sarge

THANK YOU KRIS!!!