# Land Surveyors United

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# How to correct Control points Coordinates

i have 2 control points created by GPS on my new project, and my Total station without Scale factor (Leica 407)

So, the Orientation get 16 cm difference

What should i Do?

This Content Originally Published by a land surveyor to Land Surveyors United Network

Views: 3163

### Replies to This Discussion

GPS coordinates are in grid format and total station shots will give your ground distance. you will need to establish local grid to ground conversion to establish rotation angle and distance conversion factor.

Good answer Robert,It also depends on what type of coordinates you are talking about Ahmed . You can not just say GPS coordinates explain what kind. How they were obtained and by what method. And what do you mean by the TS is without scale factor. It must be set to 1 or is this what you mean or you referring to projection scale factors.Plane projections have different scale factors. too much missing information. Are you using a collector on the total station. That is over 0.50 ft  And how are you using GPS coordinates with your Total station are these local. Did you inverse these distance to compare two different types or you comparing the same type of coordinates from each method.

ok

Dear Ahmed:

GPS is working with UTM coordinates on WGS 84 ellipsoid, so the coordinates you are getting from a GPS has a factor scale in UTM P.S. When you are using a T.S. without factor scale, you have a "pure distance" that can not be compared with UTM coordinates unless you applied the factor scale.

So, first of all, you should place 4 references (with 2 references you can not check, with 3 references you can check the distance and orientacion from one reference, but not the closing error, with 4 references you can check all of them) and observe it with GPS. You will get the coordinates as well as the factor scale for this point (If you can´t get it, there are a lot of excel datasheets that can calculate it... google it).

Then, for any observation between this references, you should consider an average of the factor scale. If you don´t want to keep all this data, and the distances between your references is not soo big, then you can make an average of the 4 factor scale and introduce this average in your total station.

In order to have an idea of the deviation you will get, consider the biggest and smaller value of the factor scale, take the difference to the average and multiply by the distance to that point... you will get the expected approx. value of the "error" you will get.

If you need some help, you can contact me at my email.

Regards

David good advice you gave him , But i still do not understand how you can run a total station with no scale factor set, it cannot be set to zero .the factor in the instrument has to be set on something. you cannot multiply by zero the coordinates can have a scale factor in the matrix .but the instrument settings to default the scale factor is ( one)  you can change the scale factor for your coordinates in your programming but you cannot change the scale factor on what distance the instrument is shooting ,you can on what is being recorded and scaled for UTM WGS 84 ellipsoid .  You may be talking about the scale factor for different recording mode or programming  but there is no setting to set the instrument to a zero factor unless you two of you are referring to no factor, when it is set at 1.But you must understand if you are saying there is no scale, it is always on 1. in normal operating mode, are it would not work .Total stations only shoot a slope distance, then this is converted to a horizontal distance using  the zenith angle or a vertical angle some instruments have both.and there are only two kinds, its a slope distance or a horizontal distance  any thing else is a projection.Please describe to me and show me the definition of where i can find this" Pure distance" this is new to me.

billy

David:-  you helped me much. many many thanks

Billy :- i meant  that i can not change the scale factor in my old TS that Leica stop making it 10 years ago.

also its mean that the scale factor is on ((1)) always and changing it affect the ppm accordingly Zenith.

any way local coordination will be enough for me.

also i will try to make 4 BM and close traverse to correct the errors.

Best Wishes

billy

http://sgss.ca/

Billy sgss trial version have only limited option ..........very good program but trial version have only limited access

Thanks Billy

Dear Ahmed:

In your TS Leica 407, go to EDM Settings, then choose Scale, and then place the factor scale that you got from GPS. It should be a valor between 0.9992 to 1.0006.

Your TS model have it... (I´m in this bussines since I was 6 years old, so 36 years, and the 25 years old TS have it... all of them!!!) The big issue is where is it and how it is called.

If you can't find it, it is not a big drama. Take all the data you can,  and once that you finish, export to a CAD file, try to find a point in the middle of the area you are working in, and apply a factor scale that comes from the average of your 4 bechmarks. And then... UTM has been applied!!!

As it is a conform projection, the angles are the same, the only change is the distance, and on this way you are aplying it.

If you need to stake out anything on this area, keep a small table with you like this... (Applying your factor scale, I will consider 0.9996).

Horizontal Distance                     Real Distance

1 meter                                      1 meter

5 meter                                      5.002 meter

10 meter                                    10.004 meter

20 meter                                    20.008 meter

50 meter                                    50.020 meter

100 meter                                 100.040 meter

200 meter                                 200.080 meter

So every time that you are reading in your TS something that is 200 meter away of you, you should keep in mind that by UTM proyection it is not 200 meter away from you, it is 200.080 meter away from you!

If it is better to work at reverse then, instead to divide by 0.9996 (or your Factor Scale), then multiply by 0.9996.

Is it useful for you?

NOTE: If your area is bigger than a couple of km or you are near a UTM huso change, then you should split your area in 1-1.5 km packages, and apply a factor scale to everyone.

Mr. David García Bodego I wanted to ask if I have control points taken by GPS in WGS "geographical coordinate system" and then converted to local "projected" coordinate system using GIS software do I need to made any adjustment to the readings I take from Totalstation

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