Two survey sessions can have the exact same number of visible satellites and produce very different accuracy, because accuracy depends on where those satellites sit relative to each other, not just how many there are. This is measured as Dilution of Precision, or DOP. This tool lets you drag satellites around a sky plot and see DOP change live, the same effect that happens in the field when trees or buildings block part of the sky.
What DOP Actually Measures
DOP is a multiplier that amplifies whatever ranging error exists in the raw satellite signals. A PDOP of 2 roughly doubles your positioning error compared to perfect geometry, while a PDOP above 6 means the geometry itself is working against you no matter how good your receiver is. Satellites spread evenly across the sky, high and low, north and south, give strong geometry and low DOP. Satellites clustered together, or all confined to one part of the sky because of obstruction, give weak geometry and high DOP.
| PDOP Value | Rating | Field Implication |
|---|---|---|
| Under 2 | Excellent | Ideal conditions for RTK fixed solutions |
| 2 to 4 | Good | Reliable for most stakeout and control work |
| 4 to 6 | Fair | Usable but expect wider scatter, consider more observation time |
| Above 6 | Poor | Avoid recording control points, wait for better satellite geometry |
Why This Matters On a Real Site
A tree line to the south or a building wall on one side does not just block a satellite, it removes an entire region of the sky from your geometry, which is exactly what the "Under Canopy" preset in the tool simulates. This is one of the core reasons DGPS RTK struggles under forest cover regardless of signal strength, the geometry itself degrades even if a few satellites are still visible.
Open-sky setup gives the receiver a full hemisphere of satellite geometry to work with.
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