PDOP, HDOP and VDOP
What are these values?
How do they work?
This article provides answers to all that and the reasons why GPS/GNSS users should have been ignoring them for awhile now.
The simple answer is there are a lot of satellites available in the sky today and today’s GNSS receivers provide an estimated horizontal and vertical accuracy for us.
The full explanation is as follows:
DOP stands for Dilution of Precision. Dilution of Precision is a term used to describe the strength of the current satellite configuration, or geometry, on the accuracy of the data collected by a GPS or GNSS receiver at the time of use. Thus, PDOP is Position of DOP and can be thought of as 3D positioning or the mean of DOP, and most often referred to in GPS; HDOP is Horizontal of DOP; and VDOP is Vertical of DOP.
GPS and GNSS receivers communicate with the satellites above to triangulate our position. Satellites are very good at triangulating our horizontal position, and less accurate at vertical positions. This can be thought of in the similar way our phone communicates with cell towers to roughly triangulate our position. With GPS receivers, when satellites are grouped together in the same general area of the sky, the satellite geometry is considered to be weak (higher DOP value). When satellites are evenly spread throughout the sky, their geometry is considered strong (lower DOP value). Thus, the more satellites available spread evenly throughout the sky, the better our positional accuracy will be (and the lower the PDOP value).
Older GPS receivers were not equipped with accuracy algorithms to estimate the horizontal and vertical accuracy of the data being collected. Because of this, we were trained to watch our PDOP values with the rough idea that values below 6 were good enough and values below 4 were great. Values at 9 or higher meant that the user shouldn’t rely on the accuracy of that data and should wait until a better PDOP value could be attained by the satellites moving into preferable positioning in the sky (or spreading out). If you want a reference to the values, Los Alamos National Laboratory has an old GPS Guide you can review.
Personally, I remember using Trimble Geo handhelds in the mid-00’s where for a whole summer the PDOP value floated around the 9 range from about 11:30am to 1pm every day, with better values in the morning and late afternoon. Luckily, those days of poor PDOP values are long gone with the advent of GNSS receivers that are capable of tracking GPS and Glonass satellites and the addition of more satellites. The better GNSS receivers today can track more than 2 satellite constellations, giving them access to many more satellites simultaneously. Because of this, in practice, we rarely see PDOP values greater than 4 for work in the continental U.S.
Another reason we should be ignoring PDOP and focusing on estimated accuracy is that PDOP values can be misleading. If you are working in the open, then it is likely the PDOP value is good and estimated accuracy good. If you move next to canopy or under moderate canopy, then the number of available satellites not being blocked by the canopy will go down by a certain number and estimated accuracy will decrease. However, if the fewer satellites being tracked from under canopy are spread out evenly in the sky, then PDOP values will still be good. Thus, if you only watched PDOP values, you would unwittingly be recording less accurate data but believe it was just as good as the data you were collecting out in the open.
Not All GNSS Receiver’s Est. Accuracy Are Created Equally
With PDOP defined and explained, it means that users can rely on their GNSS receivers estimated accuracy to determine if they are meeting project accuracy requirements. However, not all receivers behave the same. Each GNSS manufacturer has to come up with their own estimated accuracy algorithms. Then, responsible manufacturers test their algorithms relentlessly against known locations to fine-tune their GNSS receivers estimated accuracy output.
During Anatum Field Solutions exhaustive Bluetooth Submeter GNSS Field Test, they found some receivers to more accurately predict their accuracy than others. In their testing of the Bad Elf GNSS Surveyor, Eos Arrow 100, Geneq iSXBlue II, and Trimble R1, some results were surprising. Both the Bad Elf’s under-estimation of its accuracy and the R1’s over-estimation of accuracy. Since the publication of that article, we have had clients report similar R1 accuracy over-estimation.
Here are the findings from the Anatum field test:
Example: RP location NW Bridge. Trimble R1 screen shot on the right showed 0.63 meters estimated precision but the actual error was 3.47 meters. This is a major concern because users rely heavily on the estimated precision provided by the receiver to make decisions on when to store data and then also record the estimated precision as GNSS metadata in their GIS to make decisions at a later date. A quick summary of actual receiver performance vs. estimated is as follows:
Bad Elf GNSS Surveyor – Never over-estimated actual accuracy.
Eos Arrow 100 GNSS – Over-estimated actual accuracy 15% of the time.
Geneq iSXBlue II GNSS – Over-estimated actual accuracy 33.3% of the time.
Trimble R1 without external antenna – over-estimated actual accuracy 100% of the time.
Trimble R1 with external antenna – over-estimated actual accuracy 66.7% of the time.