RTK & Levelling for Topography Survey

When we use MoDOT (Missouri's DOT) VRS network with Geoid12A we can hit St. Louis County benchmarks within 0.02'. Yes we use it a lot.

> I have been relying on my RTK elevations more & more & more for my topography surveys.
> ...I do several RTK readings for traverse stations tat are under open skies. ...
Several readings one after the other, under the same satellite geometry, probably doesn't do much for you. 2 readings, separated in time by at least a half hour would be much better. Give the satellites a little time to travel.

>...if values differ by 5-20 cm then I use the RTK values for z.
5 cm (~0.15')seems like quite a lot to me. 20 cm is definitely too much for me, for the work I do.

Getting the base in a very open position is key.

I believe that repeatable elevations at the 1-2cm level are achievable with RTK alone.

I find it totally feasible. I check in with my control before starting, hit any traverse points during the topo then hit the control again when I'm done. For larger projects I hit multiple control points throughout the day and repeat as often as possible.

I, teasingly, say my middle name is "Redundent" LOL

I second this. Differences of 20cm seem huge, for either RTK-vs-redundant RTK or RTK-vs-conventional, certainly way out of tolerance for most of the stuff we work on.

> I second this. Differences of 20cm seem huge...
He may have meant "mm", not "cm". 5-20mm would be reasonable for RTK.

5-20 cm for total station elevation closure.
Most of the time, 1-2 cm is achievable with a total station for a ~5 traverse station loop on flat open terrain. But when they pass through thick vegetation & sloping land then my error of closures reach 5-20 cm. These are the loops that I recheck via RTK.
If loop passes through tree canopies then I make it a point to set a point on open ground for the RTK check.

My experience is you can't efficiently do a level loop through thick vegetation. You will also get a high error of closure.

> ... But when they pass through thick vegetation & sloping land then my error of closures reach 5-20 cm. ...
Thick vegetation and slopes really shouldn't affect your vertical closure. Failing to get your tripod's feet firmly planted in the jungle floor might. We have that problem here in the Pacific NW. Sometimes we have to dig through a foot or more of humus to get the tripod feet on solid ground.

I've used RTK to get a base elevation, the more readings the better.

I've actually run several experiments over the years, and have come to these procedures:

When setting curb and gutter, sanitary sewer, and other critical stakes, I use a total station.

If my work area entails several stations, I always level between them. I use an "old" Wild NA-28, and I always close within .01'. That gives you, and more importantly, your subcontractors, the peace of mind they need. It's all about trust.

Sleep well. 😉