We tested a unique solution to a common utility problem today. I took the opportunity to take some pictures.
The application is storing attribute data for demarcation points on homes and businesses. Lets say you are mapping the telephone demark on the north side of a building. How do you get an accurate location?
Method 1: Shoot two offset points in the open and then tape in to the demark location.
For example on the north side of our office there is a telephone demark. You could shoot point 1, then point 2 and make a note that the demark is actually 61.2 feet from point 2, along the line from point 1 to point 2.
If you are only doing one or two per day, this would be fine.
Of course if you are shooting 1000 demarks per day, it would be a lot of work in the field and a boat-load-lot of work in back in the office. A heck of a lot of work. And lots of opportunities to screw it up. (Think line from 2 through 1 on to the demark.)
Method 2: You could actually snap this from the corner locations of the building as georeferenced in on aerial photography. The image above is from maps.google.com; it is within 3-meters of being correctly referenced. Thus a distance along the south edge of the Shingleton’s building from the south east corner might be within 8 feet of the correct location.
Method 3: Just take the shot with a handheld and hope for the best. (I will show you this in a moment.)
Method 4: Big Prism Pole! Over the past five years I have sold a half dozen of these:
This is a ProMark 120 with GLONASS and MobileMapper Field. The customer will use MMField because they have lots of data to attribute at the demark and they can use the internal camera to take pictures and the microphone to take voice notes which get attached to each demark record.
MMField also gives them an opportunity to add post-processing if they need better than the 1.5′ real-time accuracy and there is no RTK network available. Since the PM120 includes RTK and NTRIP options standard, you can make a network connection if they are available.
We custom make the antenna cables so they are the correct length when a 15’ prism pole is fully extended. It makes a neat package.
Now it is not perfect:
- A 15’ prism pole weighs 14 lbs
- The receiver bracket has to go on the bottom of the second extension.
- There is a real risk of electrocution if you move with the mast up.
But there is NO LOSS of accuracy when shooting GPS positions in IMPOSSIBLE locations. Like this:
The antenna is actually above the gutter/roof line of both buildings (and our building to the south is 14.7′ tall!)
Here are screen shots of the position and SV plot with the external antenna:
While the PM120 with GLONASS kicks butt in a crazy bad location with the internal antenna, there is no loss of accuracy at all when using the elevated external antenna. And the Big Prism Pole works when the buildings are metal too.
In case you are wondering, here are the displayed coordinates:
40 44 10.41048 N 111 51 33.78588 w nad83; ExtAnt
40 44 10.42980 N 111 51 33.75420 w nad83; IntAnt
The Internal Antenna measured 3.1 ft South-51 deg-West. Which is pretty amazing considering the awful GPS environment.