Hi, I have recently been using the volume calculation to determine the water capacity of some freshwater waterholes. I have noticed that when the Volumes are calculated it gives a maximum depth without the keel offset adjustment. eg. when viewing the edit panel in my user map, the track data gives a depth range of 0.65m min to 6.85m max. My offset was 0.25m and lowest recorded depth was 0.4m. In the Volume calc results it gives the minimum as 0.0m and maximum 6.6m, average 1.5m. Obviously I am using a bounding shore line for the minimum zero depths. It looks as if the volume calc is not including the offset. This will make a reasonable difference to the volume if it is missing the top 0.25m of the water. Any help appreciated. I am new to this this so maybe I am not doing it correctly?

Hi Craig,
The volume calculations are done using the generated map data, which is always produced with all offsets (keel + tide) applied, so the differences you are seeing do not come from this.

There are, however, some differences that you might see between the min/max depths for a generated map versus the depths shown in the volume module.

The process for map generation looks like this:

grid depth data points -> triangulate -> interpolate areas between points -> smooth -> (volume calcs) -> downsample -> create contours -> downsample again -> create 3D model.

Note that volume calculations take place before down-sampling (creating a lower resolution grid for contouring/display), and that the minimum/maximum values for depth are calculated at this point. This gives (in theory) more accurate results for the volume calcs than the result you see for the finished map, although the differences should be small.

I am not clear on whether the 6.6m maximum you mention is an adjusted depth or not, but a maximum calculated depth of 6.85 m could make sense in this situation because the keel offset is added to the recorded depth, to account for the depth of water between surface and transducer. This maximum depth could be a local maxima which was lost in the downsampling, which is why you don't see it in your map depth. The zero upper depth also makes sense when you consider that you have a zero depth shoreline that ultimately forms part of the map.

Hope that makes sense,
Matt

Hi Matt

Thanks for the reply. I've played around with the raw track data a bit more and now understand exactly what you have described. Maximum logged depth before offset was 6.6m. With keel offset, 6.85m. This was a single track point in the log and easily viewed in the track asset viewing panel. In the Project map this value is maximum value is 'lost' in the down sampling as you have described, after the grid creation. Thanks for the wise words.

Couple of other points on what is an excellent program I am still learning.

1. I think it would be good to easily create a boundary file from the outline of the 'shore' as detected and shown on the sidescan log. I realize that most people use this stuff in larger areas where defining the shoreline isn't a real priority.

2. Is there an easy was to 'add' or 'insert'in extra boundary points to an existing map boundary? I have found it difficult to edit a boundary outline after it has been initially created.

Cheers

Craig