Being in the US and working for a number of different Transportation Departments, they will provide a "surveying book" that sets out the procedure for control layout. If you're not working with the local Transportation Department you might still look up if there is a book or at least some guidelines how to do this and follow it.
I also looked at TBC and entered the Irish TM75 datum into a file and put two points in it more or less in central Ireland. I used 100m for an elevation. Then I inversed between two points 1000m apart in grid and got a ground of 999.972m or three cm shorter. This means the ellipsoid is above ground by quite a bit. You will need to be around 900 meters in elevation to get ground/grid to mesh.
I might not be on the correct projection, it's flipped from what I'm accustomed to seeing.
Plus seeing geoid heights of 187meters is quite something.
Still 3 cm in 1000 is 30 pmm which will probably be OK for building your project (assuming I'm using the correct projection and somewhat near your project). Lat N53-48-30 Long W7-37-30.
I would run control exclusively in grid setting out control quality monuments, then decide if the project stays on grid or it needs to be on ground.
At that point you have some options how the ground coordinates will be projected. I can't see a need to change it to ground with that small of a ppm "error" but if it is changed to ground don't ever change anything once you do it. Keep very good records of how it's projected to ground.
As others have said in Trimble there isn't any need to set a correction in your TS, it's all controlled by the DC and TBC. After control values do the new ground coordinates and set the projection into TBC, load it onto the DC and everything follows from there.
Produce a spreadsheet with control points, description of the monument, description of how data was collected (S, RTK, FS, L, ect.), ground coordinates, grid coordinates, lat, long, ellipsoid height, orthometric height, projections used, scale factor if ground is used and the typical job name information.
Brilliant thanks. It is actually for the National Transport Authority in Ireland, but surveying over here isn't as regulated/structured as in the US, I think. We get a project specification that asks for a minimum of 6 permanent control points, required accuracy and that is about all.
In relation to location for inverse scale calculation we will be on the east coast 53.306334,-6.235656 which I think is closer to 1.
It's certainly not my call, but if grid is less than 30ppm from ground then it would be an easy decision for me and I would apply grid and keep everything on grid. Approximately a 6meter elevation change causes 1ppm change in the scale factor. 60 meters will push it to 10ppm, With GPS and modern computer systems a surface needs to be picked to survey on depending on the elevation and grid scale.
For surveys like yours finding the perfect scale factor to bring up (or down) grid to ground is impossible, there are always enough changes in elevation or easting to add ppm as you survey. Reducing the ppm is the goal and if it's small anyway there isn't a need to chase it around the project.
When figuring out a site scale factor we simply look at Google Earth and decide where the high and low points are. If there is 120 meters change in the site that will organically create 20ppm, so I will mean that between the high and low to keep the outliers within 10ppm or 1cm in 1000m. For TM projections the easting will have an effect also; but not as much as elevation usually, unless it's a flat site running a long distance E-W.
Since you're surveying with Trimble all this should be seamless, set grid in TBC, export it to the DC's and it's all "hands free" from then on.
