Hello Everybody,
I am new to this community...I have a question regarding Trimble Business center...I have a road project which is 110km in length...I have established control and processed the static data ...I want to convert my Grid Coordinates to ground coordinates...But in TBC the option of local site settings allows me to use one scale factor....But the scale factor in my project area is changing very quickly .... can i use different scale factors for different control points??
Or can i calculate the new Ground coordinates in Excel using different scale factors??
A prompt reply will be highly appreciated
I often work on large projects that can reach over 100 miles. I never try to use multiple scale factors in TBC. I split the projects up based on scale factor limits. The data always stays grid in TBC and the conversion happens in the CAD software and each drawing is named accordingly. This works well for me as most of the field data is collected in GNSS. For the TS work it is easiest to reduce to GRID once the collection is complete, then add to the mix. The hardest thing on large scale projects is to keep all the grid data with grid and try with all your might to not mix grid/ground.
Your options include
1) Using an average scale factor and accept the distortions.
2) Use a grid system specifically for the project that has small variation in scale factor (also known as a Low Distortion Projection)
3) Give setting-out in grid co-ordinates and give the operatives scale factors to put into their total stations according to location. If the setting out is being done with GNSS the scale factor will be used automatically.
I would probably go for 1 or 2. I would not like to risk 3 if total stations are to be used for setting-out..
Given the length of the project, probably 2). Make sure the grid co-ordinates look completely different to any of the other grid systems in use.
No. 3 may work fine. I know Carlson's SurvCE will calculate the proper combined factor based on each instrument setup. The user enters nothing, except a toggle to reduce distances to grid. I suspect other data collectors will do this as well, I'm only familiar with Carlson in that regard though.
If the combined factor is changing substantially with the Grid (you didn't specify what the Grid was), it may change a lot with a low distortion projection too, unless the Grid type is wrong for the project orientation (e.g. State Plane is Lambert and the project is North to South, or State Plane is Transverse Mercator and the Project is East to West). Because the combined factor is determined by the Scale Factor (distance from the Grid surface to the Ellipsoid) and Elevation factor (distance from the Ellipsoid to the ground surface), a different projection can reduce the scale factor if it is designed to suit the project orientation, and it can reduce the elevation factor by creating the projection surface closer to the average ground elevation of the project, but it can not address severe changes in elevation across a project. If this is the cause for your combined factor variations, a single LDP will not be an effective cure.
Here's the thing most of us keep missing with regard to combined factors. In reality combined factors are to be applied to measurements, not coordinates. This is because the combined factor changes from one place to another, which means that the combined factor is not homogenous across most survey projects. Now if the combined factor is fairly consistent (a subjective condition) across a project then scaling all of the coordinates is a functional solution. If the combined factor is not consistent across the project, then this solution is no longer acceptable, but it was never really technically acceptable anyway, it was simply a functional shortcut to get ground-ish inverses from grid coordinates. So with this short-cut being out of play, you are back to more technically rigid solutions. Squowse gave some good options.
It's possible that a custom projection would suit your project with an acceptable single combined factor for the entire project. Elevation differences and project extents will determine this. You mentioned 110km. If it's a corridor that mainly runs in a single direction then applying the right projection type will be helpful. You could also create several LDP's that cover the project.
Similarly, you could articulate the project with several zones with a scale factor for each zone. As Squowse mentions, you'd want to change the coordinate values substantially from one zone to the next to remove possible confusion, although I think this is going to be a pretty confusing solution anyway
Personally, I'd look at the LDP solution, but I am familiar with creating Low Distortion Projections. If that was not a good solution, I'd stick with the Grid (whatever it is) and use the appropriate Combined Factor at each setup (provided the field software allows this).
Yes good total station software will calculate the appropriate scale factor for you, if it knows the projection.
What concerns me is that a subcontract crew (for example) turning up and using the control co-ords without knowledge of the requirement to use the projection or local scale factor. In this country, at least, that could happen depressingly easily.
Just wondering - is there definitely no way that an LDP can smooth out scale factor problems caused by big elevation differences? Not a problem we encounter here very often.
Salman Mukhtar, post: 388638, member: 12052 wrote: Hello Everybody,
I am new to this community...I have a question regarding Trimble Business center...I have a road project which is 110km in length...I have established control and processed the static data ...I want to convert my Grid Coordinates to ground coordinates...But in TBC the option of local site settings allows me to use one scale factor....But the scale factor in my project area is changing very quickly .... can i use different scale factors for different control points??Or can i calculate the new Ground coordinates in Excel using different scale factors??
A prompt reply will be highly appreciated
You may want to take a look at this thread.
squowse, post: 388659, member: 7109 wrote: Just wondering - is there definitely no way that an LDP can smooth out scale factor problems caused by big elevation differences? Not a problem we encounter here very often.
The only exception that I am aware of would be designing a projection surface that deflects from the ellipsoid at about the same slope as the terrain. But even then, you're looking at a smooth surface, not an undulating one. For the record, I've never designed an LDP like that and I don't know how feasible it would be.
squowse, post: 388659, member: 7109 wrote: Just wondering - is there definitely no way that an LDP can smooth out scale factor problems caused by big elevation differences? Not a problem we encounter here very often.
The only exception that I am aware of would be designing a projection surface that deflects from the ellipsoid at about the same slope as the terrain. But even then, you're looking at a smooth surface, not an undulating one. For the record, I've never designed an LDP like that and I don't know how feasible it would be.
With a Trimble Access data collector simply use the grid coordinates and select the appropriate projection and zone and geoid file. If the design points have zero elevation then you need to enter a project height in the project setup. If you assign elevations to the design points then the software will do the rest.
I can tell you that DOT changes scales about every 10 miles east-west in a TM projection. I have a client working on a north-south project that changes scales at each township crossing or about 6 miles, this is a Lambert projection.
If you need ground there aren't lots of choices, If you are running north-south and it's a TM projection you will be limited by elevation changes but you can push the scale a long distance, same for a east-west project in a Lambert
Shawn Billings, post: 388665, member: 6521 wrote: The only exception that I am aware of would be designing a projection surface that deflects from the ellipsoid at about the same slope as the terrain. But even then, you're looking at a smooth surface, not an undulating one. For the record, I've never designed an LDP like that and I don't know how feasible it would be.
Yes that was the lind of thing I was thinking of. I really don;t know enough about it but if the projection surface was tilted you might be able to reduce the elevation factor variations??
There is a practitioner here that will devise a "snake grid". For å£å£å£. http://snakegrid.org/about-snakegrid
I believe this adjusts for elevation effects as well. As I said this will not generally amount to much in this country.
