Over the past few weeks, I have been inundated with calls from experienced users who are setting up GPS bases on sites and blowing the process. [ See also ]
There is a common misconception that you can put any coordinate in the base and have it work well.
These misconceptions are promulgated by manufacturers who hide automatic localizations behind a magic ‘setup’ curtain. A user can push a few buttons, configure a job at a site and sometimes end up with good coordinates, but they have NO IDEA what is going on in the background. While the underlying true base position and the automatically generated projection and ‘calibration’ can be viewed, most button pressers have no idea what is going on, where to look for the configuration and the magic site calibration ends up existing ONLY in the data collector of the ignorant button pusher.
If you are going to use RTK GPS, you need to explicitly understand what is happening when you configure a base.
I thought that I would write a detailed description of my thoughts on base configuration, but have re-read the Carlson User Manual and found it to be excellent. I have decided to make a few additional notes over the next few days here on the blog.
You can download a PDF copy of the User Manual using this link: [ SurvCE User Manual ] or from the Carlson Website. The Base Configuration section starts on page 146, is not too difficult of a read, and is an excellent resource.
Let me add these thoughts:
- When you configure the Base, your goal is to tell the GNSS base what the actual coordinates for the electrical phase center of the GNSS antenna are, including the actual ellipsoid elevation.This is done by telling SurvCE the position of the Ground Mark. (The point on the ground under the GPS head.)If you specify a projected Grid Coordinate, then SurvCE will convert it to the equvalent Lat/Lon/Height.If you specify the Orthometric Height, SurvCE applies the GEOID and computes an Ellipsoid Elevation.
Since you have provided the Ground Mark elevation, SurvCE will add the rod height HI (either slant or vertical) and the L1 offset (determined by the antenna model ) which sets the offset from the bottom (or measure up mark) to the electrical phase center of the antenna.
You give SurvCE the Ground Mark position and SurvCE computes the electrical phase center of the GNSS antenna on top of the pole and sends this Phase Center position to the receiver with a command to ‘Be a Base’ and broadcast corrections.
This Phase Center position HAS to be within 100 feet X,Y and Z of the TRUE geodetic location. The Phase Center elevation HAS to be ELLIPSOID so you HAVE to load and use a GEOID file if you are going to enter orthometric heights.
If the position is in error by more than 100 feet then Rovers which are using the Base for corrections probably will not be able to fix, or will fix extremely slowly. Bad base initializations also can have the effect of making the base appear to ‘hang’ — it just can’t compute correctors for the signals that it is receiving to broadcast.
Even the difference of the Geoid height (typically 30 to 60 feet) will severely hinder the rover’s ability to fix.
- I am not going to sugar coat the following statement: “If you are initializing your Base with an Orthometric height then you are crazy.” This is a cardinal sin and it cannot be forgiven.Let me give you an example. Pretend that you are at this location:
Lat: 40 44 10.22058 State Plane Northing (UT C NAD83): 7437104.68825
Lon: 111 51 34.19263 State Plane Easting: 1540786.56835
Ellipsoid Height: 1306.483 M (4,286.353 ft)
Orthometric Height: 1323.212 M (4,341.238 ft)
You might incorrectly configure your receiver as shown:
In the screen shot above, the orthometric height is entered as an ellipsoid height. This results in a 55 foot bust in the broadcast position of the base. This will significantly slow down a Rover’s ability to fix.In addition it will result in bad orthometric heights as the distance from the Base to the Rover increases.(The reason the ‘Orthometric’ option is grayed out is no GEOID is loaded or available.)
- An important statement in the User Manual is: “When you are starting a new job (no information in the raw RW5 file yet), always use the options in From New Position.”
That sums it up: if you are starting a new job or series of jobs, always use the ‘From New Position’ tab. If you have coordinates for the base that are accurate, then go ahead and use them using the ‘Enter Lat/Lon’ or ‘Grid System Coordinates’ button.
Otherwise use the ‘Read GPS’ method of getting an approximate position. (You can always set the base at a random position, Read GPS and then do a localization on the Rover.)If you are reoccupying a base point that you have occupied before, then use the ‘From Known Position’ tab. The most common button is then ‘Read From File’.
- The Carlson User Manual says that ‘Read GPS’ will return a position that is accurate in the range of 10 to 100 meters. On modern GPS/GNSS receivers, if the receiver has been turned on and tracking satellites for 5 minutes (and is located in the USA), a read GPS will return coordinates that is within 0.3 meters (1 foot) of the TRUE IGS08 framed location.The IGS08 framed position will be within a couple of meters of the NAD83 position that you probably want.
I am working on a follow-up post
- Configuring a base at a random point near your control at a job
- Using a single point calibration to match control on the control point (with the random base)
- Setting up the Base on the second day at this job
So more on this in a later post. ==> [ See also ]