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?
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.
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.
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.
