Data Offsets

Used to manually add a data offset to be used in the calculation.

Data offsets are a critical part of maintaining highly accurate bathymetric and vegetation data. An offset can be used for anything from correcting for the depth of the transducer below the water's surface to identifying depth based on a high or low water mark.

Note: Data offsets are always applied to the original data and each time a value is adjusted,the trip is queued for reprocessing.

Manual depth offsets

Bathymetric and biovolume outputs can be adjusted to defined depths. These adjustments may vary from offsetting outputs to the depth of the transducer or to adjust outputs to a standard or simulated water elevation.

For example, some users may want to set outputs to a standard ordinary high water mark or pool elevation. Establishing a benchmark water elevation and applying the appropriate offsets to trips prior to merging is critical for trips that span time periods where water elevation has changed significantly.

Note: Offsets are applied uniformly to the entire file so if you are mapping in a non-tidal system that changes elevation relatively rapidly, then we recommend saving short files, applying the necessary offset, and then merging the multiple files.

Positive vs. negative offsets

Positive offsets will uniformly make depths deeper, while negative offsets will make depths shallower. Simply add offsets to account for both the transducer and water elevation (e.g., if your benchmark elevation is 1 ft lower than the depth on the trip date, but your transducer is 1 ft below the surface, then these values cancel each other out and no offset is needed).

Note: Positive offsets will not “spill over” the banks of the shapefile boundary used in themapping process.

Biovolume is also adjusted with offsets. For instance, biovolume will increase (i.e., the heat map will get hotter) if a negative offset is applied (or if tide offsets bring the water elevation down) because the plant length now occupies more of the water column. With a negative offset, if the water depth decreases beyond the plant height, then biovolume will be 100%.

With a positive offset, biovolume will always be less and thus be “cooler” in the biovolume heat map. For example, a 1 ft positive offset will reduce a 100% biovolume reading in 2 ft to 66% (i.e., plant height in this case equals 2 ft and water depth now equals 3 ft). This same offset applied to a 100% biovolume reading in six feet would result in an adjusted biovolume of 86%. Consequently, this results in an arc-like pattern of maximum biovolume values in the summary reports.

Note: Offsets cannot be applied to merges and must be applied to individual trips prior tobeing merged.

Automated tide offsets

One of the biggest challenges of mapping coastal habitats is their tidal influence with depths changing harmonically based on the moon phase and other factors. Fortunately, however, widespread tide stations and large public databases of tide predictions allow for accurate and precise offsets to georeferenced and time-stamped sonar logs uploaded to EcoSound. EcoSound periodically queries the nearest tide station to your upload (up to 75 km) and adjusts your depth and seagrass or kelp biovolume to the Mean Lower Low Water (MLLW) datum.

Note: The frequency of offset queries depends on the tide station. To confirm the frequency, export the CSV point data of trips influenced by the tide and examine the timestamps.

Note: When in a tidal estuary with a large and rapid tide there will be a lag depending on the distance from the tide station and the characteristics of tide. In these cases, we recommend removing the Tide Offset and applying an average tide offset manually to the file based on local knowledge of the tide conditions at the site. Again, save files in short "bursts" for maximum accuracy.

The tidal statistics are associated to the selected trip in the BioBase trip viewer and summarized at the bottom of summary reports.

The image shown below is an example of a Data offset dialog that shows the automated tide offset to Mean Lower Low Water (MLLW) combined with a transducer depth offset on a selected trip. In this instance, depths were decreased on average by 3.5 ft to represent the state of habitat at MLLW tide.