Ground control missing elevations


#1

Hello all,

Haven’t posted in awhile, hope everyone in the rest of the US is thawing out. We are finally seeing some ground here in SC.

Tomorrow, I want to map a new, arrow straight section of highway (about 4,000) for as-built documentation to use for preliminary planning of adjoining infrastructure. No traffic, rural location. For control I have new water valve boxes and line marker monuments. Good targets, the problem is that I don’t have elevations, only horizontal coordinates. If I assign a static elevation to all control points, will I get a good map for horizontal use only? I realize vertical will be off but really don’t need it for my purposes. Land is very flat. Elevations at grade I wouldn’t expect to vary more than 2 or 3 feet; however, location markers extend several feet above the ground (concrete monuments). With top elevations above grade, would lack of an elevation shot present additional problems?

Thanks to all in advance for any input,

Steve


#2

Hi @StevenB,

I have seen accurate reprocessing with only XY values in the past. Without the Z values included the RMSE of the map will appear to be way off while the X and Y values should be fairly accurate. I generally don’t recommend this but It can be done. What type of GPS are you using that doesn’t include the Z value for each coordinate?

When you indicate that the highway is “about 4,000”, is that referring to miles? Feet? I look forward to hearing from you.

Cheers!
Zach


#3

Thanks for the info Zach. I was delayed on my flight by weather but hope to map the location soon and will post results. Length is 4000 feet. Reason that I don’t have Z coordinates is that I am referencing an as-built drawing that shows only x,y on existing above ground water line appurtenances. As I said, this is only for preliminary planning so highest precision is not critical. In our area, all as-built drawings are required to have coordinates (sewer must have z coordinates too) on visible elements. Manhole covers, water valve box tops, cleanout tops, line location monuments make convenient and easily visible targets and provide a easy way to get a reasonably accurate map with GCP for preliminary planning purposes as long as one understands the limitations. Your new GCP tagging process is a real time saver because I no longer have to position targets over the GCP item. I know what the manhole or valve box looks like so I can easily tag it myself without a target. If we need an accurate map later, we can reconfirm the coordinates with GPS (including Z where required) and reprocess the same photo set with an updated GCP file.

Thanks

Steve


#4

Hi @StevenB,

This seems to be coming up more often with a few of our customers that carry out preliminary planning. One thing to mention is that the RMSE value of the resulting map will seem a bit high for a map that has been reprocessed with GCPs. The XY values will be more accurate than what you would expect from a traditional drones GPS. I look forward to seeing the project in the end. Feel free to post some screen-shots of the map when all is said and done. :slight_smile:

Cheers!
Zach