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How typically a Scale Factor is used on a construction site?

georam (Geotechnical) (OP)
15 Jun 09 12:56
For a large project, such as a hydroelectric development, the construction drawing will contain a project layout and control drawings, in which dimensions will be provided in true lengths, rather than in a UTM coordinates, such as NAD 83 coordinate system.
Since the topo maps are established from a UTM coordinates, a scale factor will be used so that the survey measurements (in true length) can be represented correctly in the field.
The scale factor can be calculated, based on two items: (1) location, and (2) elevation.
The location will determine the horizontal scale factor, and the elevation will give an elevation scale factor. Both will be combined to come up with a combined scale factor.
For simplicity, it would be nice to pick up a scale factor (usually to the fifth decimal, such as 0.99922) for the whole project site. This would be a combined scale factor utilizing the location and elevation.
My question is as follows:
Assuming a project site about 2-3 square miles. We can assume that the coordinates within this area will generate a single horizontal scale factor, relatively unchanged to the fifth decimal.
How do you deal with the elevation? Should you use an average elevation, pick up max and min and average them, or by other means? This is assuming that elevation within the project site will not allow to pick up a single elevation scale factor to the nearest fifth decimal.
Just for (1) and (2), is the common guidelines just to pick up an agreed combined scale factor?
I would like to have a discussion or comments please
Thank you
IRstuff (Aerospace)
15 Jun 09 14:13
Is there no distinction between NS and EW distances, then?  Isn't there more error in the EW distances for UTM, depending on your latitude?

But, with the availability of relatively cheap and powerful computers, ala laptops or PDAs, why not just do direct coordinate conversions between true distances and UTM?  Seems to me that a simple spreadsheet ought to be able to crank the coordinate conversions directly.  I'm a bit surprised that a total station doesn't do that already.


FAQ731-376: Forum Policies

cvg (Civil/Environmental)
15 Jun 09 14:51
suggest your surveyors convert to a grid system prior to producing the plans. Topo maps should be projected to the grid system. The grid should be tied to local survey control monuments and project benchmarks (TBM) set for the project.  
Apakrat (Civil/Environmental)
15 Jun 09 16:52
In my day, Surveyors gathered  information for Engineers to produce designs for Draftsmen to put on paper.  Surveyors then use these plans to lay out line and grades for construction people!

Usually, Office Design People apply State Plane Coordinates exclusively on drawings to give the appearance of a cleaner drawing when issued.  (Sure makes a messy drawing when markings for field usable dimension lines are applied.)

Usually, local elevation differential allows establishing a horizontal correction factor which can be used throughout the project.  Another is the acceptable difference between a desired plan distance and the constructed finished product distance.

At 74th year working on IR-One2 PhD from UHK  - - -

tony45power (Civil/Environmental)
16 Jun 09 22:10
i work on a lot of big project. Big and small though i work with Carlson survey program.  Its based on auto cad. when i get the cad DWG from the general contractor i scale the drawing to 1/12 or .083333 and then can create my 0,0 also  the control i establish in the field i place on the drawing. then transfer all my points into a data collector.  

underhill (Civil/Environmental)
25 Jun 09 11:34
"Is there no distinction between NS and EW distances, then?"

No, a UTM projection, like most (all?) projections used by surveyors, is "conformal" -- which has the very useful property that, at any given point, projection scale factor is the same in all directions. (The other property, also useful to surveyors, is that angles between lines are preserved when projecting between the globe and the map.)

What you're probably thinking of, irstuff, is the "convergence of the meridians" -- the fact that the further north (or south in S hemisphere) one is, the closer the meridians are to one another.

As to georam's question: i think that if your site's relief is such that elevation factors will differ by more than your tolerance (5 decimals), then can you not divide the site into a few elevation zones and choose appropriate factors for each zone?
IRstuff (Aerospace)
25 Jun 09 11:59
So, is the scale factor a compromise between NS and EW scaling errors?  How does one reconcile the lack of actual NS scale factor error, with an actual EW scale factor error?


FAQ731-376: Forum Policies

underhill (Civil/Environmental)
29 Jun 09 15:28
"is the scale factor a compromise between NS and EW scaling errors?  How does one reconcile the lack of actual NS scale factor error, with an actual EW scale factor error?"

If you have a large area and you only use a single scale factor, then, yes, you are making a compromise. Scale factor varies from place to place, and on a UTM projection, it varies in an EW, and not in a NS, direction.

At any given spot, however, the scale factor (assuming it has been properly calculated) is correct in all directions (there's no error).


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