GDOP / PDOP / HDOP Calculator
Convert GNSS dilution of precision values into expected horizontal, vertical, and 3D positioning uncertainty.
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Understanding DOP values in GNSS
Dilution of Precision (DOP) quantifies how satellite geometry affects positioning accuracy. When satellites are well-distributed across the sky (clustered toward zenith and horizon evenly), DOP is low and measurements are accurate. When satellites are clustered together, DOP is high and accuracy degrades even if the receiver and signals are perfect.
DOP is a dimensionless multiplier. The final position error equals DOP × UERE (User Equivalent Range Error), where UERE combines all pseudorange measurement errors — typically 1-3 m for standalone GPS, 0.5-1 m for SBAS-corrected, and 0.01-0.03 m for RTK.
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The five DOP types
- GDOP (Geometric DOP) \u2014 overall 3D position + time accuracy
- PDOP (Position DOP) \u2014 3D position only, combines HDOP and VDOP
- HDOP (Horizontal DOP) \u2014 latitude and longitude accuracy
- VDOP (Vertical DOP) \u2014 altitude accuracy (typically 1.5-2× worse than HDOP)
- TDOP (Time DOP) \u2014 clock bias accuracy
When DOP matters most
Urban canyon surveys: tall buildings block large sky areas, spiking DOP values above 10. Plan surveys during windows of favorable geometry.
Planning missions: mission-planning software predicts DOP over 24 hours based on satellite almanacs. Choose low-DOP windows for critical measurements.
Quality control: automated monitoring systems reject epochs with PDOP > 6 to avoid biased station coordinates.
Multi-constellation GNSS: adding GLONASS, Galileo, and BeiDou drops typical PDOP from 2.5 to below 1.5 in most environments.
Frequently asked questions
What PDOP value is acceptable for surveying?
Most specifications require PDOP < 4 for RTK work and PDOP < 6 for DGPS. Post-processed static surveys accept PDOP up to 8.
Why is VDOP always worse than HDOP?
Satellites are only visible above the horizon, so the geometry is always more constrained vertically. VDOP is typically 1.5 to 2 times HDOP.
How do I improve DOP?
Use more satellites (multi-constellation), set a low elevation mask (10° rather than 15°), and avoid obstructed locations.
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Related tools
Complement this with our RTK accuracy calculator and satellite visibility tool.