Exporting to HAC data files
Echoview supports writing data from any (raw or virtual) Sv, TS, Angular position or Single targets single beam echogram to a HAC data file using the generic echosounder and channel tuples.
You can use this functionality to create data files corresponding to a complex transformation of the data without having to refer back to original data files. After applying several operators on a data set, it might be desirable to store the final data set, a virtual variable, for future reference as if it were raw data. In such a case, export the virtual variable to HAC, and import it again in a new EV file, where the data are treated as raw data. Export of virtual variables is licensed with the Advanced Operators module.
HAC files created in Echoview are also readable by other software packages that support the HAC format and have support for the generic echosounder and channel tuples.
To create a HAC data file in Echoview:
- View an echogram.
- From the Echogram menu, point to Export and then click To HAC format...
- Under Variables to export, select the variables to export to the HAC data file.
- Select the Write compressed tuples option if you want to write compressed data tuples to the resulting HAC file.
- Click Export
- Choose a folder and enter a filename.
- Click Save.
The HAC file will be saved to one or more files using the naming format filename_nnnnnn, where n represents an integer greater than or equal to zero.
More than one HAC file will be created:
- if the exported variable contains changes in stop depth, start depth or number of data points per ping (a new HAC file will be started at the ping where these echosounder settings were changed during recording of the data); or
- if the file exceeds 95 MB.
To view the HAC data file, you must create a new EV file or a new fileset and add the HAC data file(s).
- Following the HAC specifications, no distinction can be made between 'no data' and 'thresholded data'. Both are dealt with as thresholded, so that when an exported HAC file is added to a fileset, all 'no data' from the original Sv or TS variable will now appear as thresholded data. Upon reading the exported data, Echoview sets the start range of a ping to zero meters and determines the stop range as the bottom range of the last sample in the ping. This may result in exported pings being be shorter than the corresponding pings in the original data. Where there are no exported samples in a ping, the echogram displays no samples.
- The HAC format does not permit variation in the number of data points per ping within one file. If the variable you are exporting contains pings with varying numbers of data points, Echoview will start a new HAC file at every change in the number of data points it encounters. This may result in many HAC files where the ping geometry is changing frequently. On expanded bottom echograms with breaks in the sounder-detected bottom line the ping geometry will change at every break.
- The HAC file format and Echoview do not support negative range values. The Echoview HAC export omits samples with negative range (and adjusts the sample count of the ping) and sends a message to the Messages dialog box. The handling for single target data with negative range is to assume the ping start range is 0 m.