*Note - Available on Raw Data Extended Exports Only
HDOP - Horizontal Dilution of Precision
HDOP, or Horizontal Dilution of Precision, is a term used in GPS (Global Positioning System) technology to indicate the accuracy of horizontal position measurements obtained from GPS satellites. It is one of several factors that can affect the precision of a GPS device's location data. It takes into account the arrangement of GPS satellites in the sky relative to the receiver's position. HDOP specifically relates to horizontal accuracy and is an important metric for assessing the quality of GPS position information. It is not possible to provide absolute values for the ranges of error in distance units (metres/feet) at various HDOP values as they provide a geometric quality marker of satellites.
HDOP is calculated based on the positions of the GPS satellites that are currently visible to the receiver. A lower HDOP value indicates better satellite geometry and, thus, better horizontal accuracy. It is a dimensionless ratio, and therefore, HDOP is not measured in any specific unit.
Image 1 - Good HDOP
Higher quality satellite signals are obtained with an even distribution of satellites, enabling a more precise identification of position of a GPS device.
Image 2 - Poor HDOP
Lower quality satellite signals are obtained with an uneven or clustered distribution of satellites, resulting in a greater potential variation in positional identification.
Interpretation of HDOP Values:
- Low HDOP (e.g., < 1): A low HDOP value indicates good satellite geometry, meaning that the satellites are spread out across the sky in a way that allows for more accurate horizontal position calculations. A GPS receiver with a low HDOP value is likely to provide highly accurate horizontal position information.
- Moderate HDOP (e.g., 1 to 2): A moderate HDOP value suggests decent satellite geometry, and the GPS position is likely to be reasonably accurate. However, the precision may not be as high as when HDOP is very low.
- High HDOP (e.g., > 2): A high HDOP value indicates poor satellite geometry due to increased geometric dilution, typically caused by the satellites being clustered in a certain area of the sky or having a weak signal due to obstructions or interference. In such cases, the GPS receiver's horizontal position accuracy may be compromised.
HACC - Horizontal Accuracy
The Horizontal Accuracy (HACC) value of a GPS location is determined based on various factors and calculations performed by the GPS receiver. This value indicates how confident the receiver is in the reported GPS coordinates. A smaller HACC value indicates higher confidence and better accuracy.
Once the GPS receiver has acquired signals from multiple satellites, measured the distances to each satellite, and considered factors like Dilution of Precision (DOP) and error corrections, it calculates the horizontal accuracy value.