California Surveyors Forums

I'm thinking of getting a receiver which can give me sub-centimeter elevation on a site far from any known BM.
What brand/model are you using? how much did you pay for it? how long do you wait to collect data? what's the time frame from start to obtain the elevation?

Sub-centimeter compared to what?

I assume you don't want to mess with post-processing -- NGS has well-established guidelines for getting ortho heights from static observations -- but even at that you're probably looking at 2 cm accuracy with respect to the NAVD88 datum under good conditions.

And I assume you don't want to deal with setting up your own RTK base station, which adds equipment, time and possibly personnel requirements to the job.

RTN (e.g. CRTN, or Cansel's Trimble network, or Leica's SmartNet) will get you *a* realization of NAVD88, but datum accuracy is going to depend on network design and where you are in the network due to e.g., distance from the reference station(s), geoid model accuracy, and observation conditions. Again, you're probably looking at multiple-centimeters.

And if you're trying to match a local vertical network, you're going to have to tie into one or more passive marks in that network, regardless of which approach you take.

Getting reliable ortho heights out of the sky isn't a slam-dunk. The best answer to your question will depend on the project requirements.

As Jim said . . . it depends on project requirements.
We set up and a commie CHC iG5 static receiver https://www.igage.com/iG5/iG5.htm . . . (check with iGage for cost. they have great equipment with technical service) . . . collect data 'til the batteries run out (about six hours . . . you can go longer with an external battery). Then send in the Rinex file for an OPUS solution. (You should wait 15 days to submit your Rinex file so NGS uses their Precise ephemeris.)

All the while working with our terrestrial survey measurements while GPS is collecting data.

Crazy Phil's two cents . . . (great post, as always, Jim)

The job requirement is to tie a topographic survey to NAVD datum. Most of time I tie to a known BM, but on some occasions the BM is far, so i was thinking a gps receiver will come to help.

The job requirement is to tie a topographic survey to NAVD datum.

Absent an accuracy specification from your client, you have a lot of latitude (that's sort of a pun) in how you go about this. Both RTN and OPUS will give you NAVD88 elevations, you just won't know what the datum accuracy is. For many of the topo jobs I do, that's just fine: the client wants the datum stated, but their design work will be confined to the site, so as long as I can ensure that the *relative* accuracy of the on-site elevations meets the client's needs, I'm content to spell out the method I used to get on datum and not address datum accuracy at all. (The old-school way to characterize this is "Who's going to make a liar out of me?") It's worth noting that FEMA will accept an OPUS result for a datum tie.

It's a different story if I have to tie to a local datum realization, e.g. a city control network. If I can reasonably make that tie with a total station I will, but if the distance is sufficiently large I'll use RTN or RTK. In either case, I'll observe at least one of the local network stations and adjust my datum plane up or down to match the published values.

Yet another situation is when the client lets me know that they have to connect their gravity utilities to offsite infrastructure, in which case measurements to the existing inverts are necessary. Then I have to decide whether or not to adjust my datum to the existing published values, or stick with my datum reference and prominently note the difference between the published and measured invert values.

When it comes to topos, it all comes down to answering the question, "Where's the water going to go?" If your survey allows the client to comfortably answer that question, datum accuracy isn't very important.