First, you need to determine if you need high Absolute or Relative accuracy.
Absolute Accuracy will tell you where you are in relation to the Earth, but not necessarily give you accurate measurements from point to point.
Relative Accuracy can tell you the distance and angles between the Base and the Rover to a very high accuracy, but it may be shifted on the surface of the Earth by many meters.
You may not need high absolute accuracy if you do not intend to measure to objects that were not made by you or at locations very far apart. For example, if you took measurements of a wall one day in Spain, and another day you (or someone else) took measurements of a wall in Germany, then high relative accuracy will give you the measurements of the wall very accurately, like having a high-quality tape measure at each location, but not knowing anything about how far apart those tape measures are, or where on the globe they are. High absolute accuracy will tell you how far apart those walls are with very high accurately, and probably not so precisely how tall they are.
GPS is inherently only accurate to within 10 meters or so of absolute accuracy, but can be tricked into being more accurate because it has high relative accuracy.
If you need absolute accuracy, you need to measure from a known point, such as a CORS network, NTRIP caster, or a control point established beforehand. This gives you your location on the surface of the planet within a few centimeters, depending on how far the network receivers are to your base. You can average your location by using multiple CORS to refine the location. This will be best done post-processing in the office using RTK-LIB. Once you know your location to high absolute accuracy, you use those coordinates in the Base Mode every time you set up.
Then, your rover will give you positions that are high absolute accuracy, but not necessarily high relative accuracy.
To make sure you have high relative accuracy on the rover, you need to have:
- A fixed solution.
- AR Ratio should be stable in the high hundreds (500+) and ideally 999.
- The base correction less than a few seconds aged.
- For very precise points (sub centimeter), measurements over time, and preferably at different times of the day when the satellite constellation has changed.
If you record a point on Reachview, it will display the accuracy of the Northing, Easting, and Elevation of each point in the RMS next to the point location. This is not enough to trust unless all the above conditions are met first.
If you want to know exactly how accurate the readout is, the only way to know without any doubt is to reference it to a previously measured location because you cannot analyze a single data point. However, if the checklist above is met, you can reasonably assume the accuracy is consistent. You can confirm this by repeating measurements on a single point over time and analyze the difference.
Now, if you considered your base setup to be a rover, and another receiver provided by a network such as CORS or an Ntrip service with a published predefined location as a base, then take one long measurement at your location, you can post-process the data in RTK-LIB and find the true location of your base, then you will have a very accurate point location that you can now use as that base point. Then all measurements made from that base location will have both high relative accuracy, and high absolute accuracy as well.
