６．サーベイ（HYPACK® Max Survey）
Q. Hypackのアノテーション処理(How HYPACK® Selects Depth Info for Annotation to Echosounders)[英語]
There is occasional confusion amongst the users who try to compare the depth annotated on the echogram with the corresponding depth shown at the fix in the SINGLE BEAM EDITOR of HYPACK®. It's a little disconcerting when you are going up a slope and read a depth of 12.9 on the echogram and then load the same file up in the SINGLE BEAM EDITOR and see a depth of 14.3 at the same fix mark.
The difference you see is due to the time correlation that the SINGLE BEAM EDITOR performs when matching depth and position information.
In the SURVEY program, we have positions arriving every second and depths arriving ten times per second. In our example, there is a one second latency (delay) from when the measurement of position was made until it is delivered to the computer. For the echosounder, we have a tenth of a second latency.
Thus, when the computer clock reads 12:05:07.0, it is receiving the position from 12:05:06.0 and the depth from 12:05:06.9.
Many users generate event marks (also called "fixes") using a distance along line. For example, you may decide to generate a fix mark every 25' down the survey line. It is important to note that the SURVEY program does not perform any prediction as to when it crosses each 25' point. It just processes each position as it arrives. If it places the vessel at or beyond the fix point, an event mark is then generated.
For the annotation string that SURVEY generates for the echosounder, it just takes the last depth received and inserts it in the text string sent for the annotation.
In the figure above, the SURVEY program receives a position fix at 12:05:07 (computer clock). Since we have a one second latency, this is actually the position from 12:05:06 (Item 1). This position triggers an event mark. SURVEY takes the last depth received, from 12:05:06.9 (Item 2) and places it in the annotation string sent to the echosounder.
Later on while processing the data file is read into the SINGLE BEAM EDITOR, the program will see it has a depth at 12:05:06.0 (Item 3 in our time line) and assign it to the correct position at 12:05:06.0 (Item 1 in our time line). Although the depths read in the SINGLE BEAM EDITOR at the fix marks may not be the same as those viewed on the echogram, the important point is that THE DEPTHS IN THE SINGLE BEAM EDITOR ARE 100% CORRECT AS TO THEIR TIME AND POSITION.
A possible solution to the above problem would be for us to buffer depths in each device driver to allow the program to retrieve a depth from the time of the position measurement. In this case, the depth on the annotation string would match the depth in the SINGLE BEAM EDITOR. The downside would be that the depth annotated on the echogram would not correspond to the analog depth on the echogram at the time of the fix.