Distances along the seafloor?

[abooda1981]

Hello! If I were interested in plotting the distance between a point on the shore and a point out at sea, but I wanted the path to be not "great circle distance" but rather the path along the seabed, is there a way of doing that? I can see that there is a minimum depth, but is there a way to change it so that the minimum depth is always the seafloor?

Many thanks!

[besibo]

Hi. I'm not really sure how you would measure that. For instance in trail running, the distance is indicated as the combination of a horizontal distance (i.e. 10 km) plus the total elevation gain or loss (i.e. D+ 1000m). I'm not sure how one would compute the distance otherwise. To me, computing the distance between a point on the shore and a point at sea is the same as computing distances for trail running. But I may be missing something?

In marmap, there are currently two methods to compute distances:

• by using trans.mat() and lc.dist(), you compute the shortest horizontal distance between pairs of points. The min.depth and max.depth arguments of trans.mat() allow you to avoid specific regions where the travel would be impossible (for instance, land area for marine species). But the distances computed are still horizontal distances.
• by using dist2isobath, you compute the shortest horizontal distance between a point and a specified isobath

In both cases, the horizontal distance computed takes into account the curvature of the earth. This is why we compute great circle distances, which is especially important over large distances.

Currently, there is no easy way to compute the total elevation gain or loss along a path, but that would be a great addition. Feel free to submit some code! ;-)

[abooda1981]

The solution I've settled on is to use something like a Pythagorean triangle to say "sqrt of distance = sqrt(horizontal distance + depth change)". I can put that into a function and maybe communicate it to you by email? (I have it from last year.)

[besibo]

That should work but it will only provide very rough estimates, especially when the slope is irregular. I would prefer to include a new function that (i) computes the elevation gain or loss for each pair of consecutive cells of the bathymetric grid along the least cost path, (ii) sum up the positive values to compute the overall elevation gain, (iii) sum up the negative values to compute the overall elevation loss, and finally(iv) report the total elevation gain and loss (in meters) along with the shortest horizontal path.

[abooda1981]

Hello! I saw this a few days ago. I'm currently away at a conference but I'm actually really motivated to have a go at this next week. Will reach out if I hit a wall.

Cheers!