What is the difference between resolution and accuracy?

Many sonar systems advertise accuracy and resolution data. What does this information mean?

Depth Accuracy tells you how sure that you can be that a depth measurement is correct. If the sonar tells you the water depth is 10 metres, is it really 10.1m or 9.9m?

Depth Resolution tells you the smallest depth difference that can be detected. So if the depth resolution is 1cm, you should be able to see a 1cm "step" in the bottom of the sea.

These two figures are very different. A sonar might be able to resolve a small object (resolution), but it might not be able to tell exactly how deep it is (accuracy).

Accuracy and resolution can also be stated in range (distance from the sonar transducers) and horizontal distance. Survey specifications such as IHO S44 define acceptable limits for the vertical and horizontal accuracy and resolution of the data from a survey. It uses the word "uncertainty" instead of "accuracy", which can be related the statistical spread of measurements made. Horizontal resolution is important here, and it is related to the density of measurements on the seabed: the number of measurements per square metre. The accuracy and resolution are specified for the complete survey, not just the sonar system. The quality of the position and attitude measurement systems are as important as the sonar system for this, and the way that the system is used is also very important. A survey made when the sea is very rough will not be as accurate as one made in calm weather! Few survey organisations give "official approval" of sonar systems. All that can be said is that a sonar system has been used to generate data that has met the approval of hydrographic authorities. Bathyswath and SWATHplus have met this requirement many times in different countries around the world.

Depth accuracy and horizontal resolution are related to each other, according to the filtering that is applied to the data. Raw Bathyswath data has very high resolution: the resolution of the un-filtered bathymetry data is the same as that as the sidescan image data, which has to be very good in order to see small objects on the seabed. This necessarily means that there is a wide statistical spread of the raw depth measurements. The data has to be filtered to reduce the statistical spread of depth measurement, and so improve the accuracy. That filtering process reduces the horizontal resolution. Detailed analysis shows that, when Bathyswath data is filtered to the limit of the data density requirement of IHO S44, the accuracy (uncertainty) is better than the limit required by the specification. The "Accuracy and Resolution" section of the Bathyswath Technical Information document explains this in detail.

Some sonar systems that are similar to Bathyswath provide additional resolution information, and the Bathyswath brochure provides this same information for comparison. This information includes:

  • Measurement resolution limit: this is the smallest difference in distances that the system can measure. Bathyswath measures range as a count of sonar cycles, and so the measurement resolution limit is the same as the wavelength of the sonar. This is 3mm for the 468kHz version, 6mm for 234kHz, and 12mm for 117kHz.
  • Resolution detection limit: this is the smallest object that can theoretically be detected by the sonar. Sonar theory says that this is half the sonar wavelength, and so it is half the "measurement resolution limit" for Bathyswath.
  • Across track resolution: this is the smallest difference in range that can practically be measured, and it is related to the length of the transmit pulse: the burst of sound that the sonar sends out. It is 1cm for 468kHz, 6cm for 234kHz, and 13cm for 117kHz. So although the sonar electronics can measure differences in distances equal to the measurement resolution limit, the length of the transmit pulse spreads out the sound echoes to the size of the across track resolution.

All three measurements are theoretical limits, and conditions in the real sea and seabed will restrict the actual resolution. Most similar systems (interferometers) work the same way, and so they have similar theoretical performance limits. The actual limits to performance are related to the quality of the sonar transducers, electronics, and software, and are best judged by examining real sonar data obtained from the sonar system.