An interesting question. I cannot think of a better approach than what
you propose, but I want to point out that even then you will not get
the true (spherical surface, great-circle) shortest-distance surface
paths over the open-water segments! For that, I think you'd have to
first do the raster-based (implicitly flat-plane) solution, then try
to identify the open-water segments and replace them with great-circle
paths, which, of course, might pass over land so you would need to
iterate...

Then again, maybe ships don't follow great-circle paths... maybe
currents are important enough to modify the optimal routes?

Presumably the path restrictions will tend to be in a few common
locations for many port-port combinations: e.g., Gibralter, Cape Horn,
etc... could you create a set of such points and then compute vector
great-circle paths between these and your ports... maybe a relatively
small subset would then need adjustment, which perhaps might be
feasible in an interactive way by adding additional points to the set
of constraints ... basically your routes would be one or more
great-circle paths from port A to (zero or more constraint-points) to
port B?

A whole other way to go... have you tried to find any kind of
published route map? Surely the shipping industry already has them all
pre-digested...

I look forward to hearing others' suggestions, and/or your solution!



On 11/10/2014 9:13 AM, Milan Budhathoki wrote:
> Looks like email that I sent yesterday didn't go through. 
> Here it is again: 
> 
> Hello Listserv,
> 
> I have point dataset of ship trips from one port to another. I want to
> calculate the shortest distance between each port pair  *over water*.
> There are thousands of voyages, and 5,000 unique ports from all over
> the world. One of the approach I can use in ArcGIS  is to run the
> Cost-Path tool having water/land as a cost raster to make a path only
> on water. But I assume that the Cost-Path approach would be little
> tedious for a large dataset. I wonder if anyone in this forum has a
> suggestion to calculate a shortest distance between two points having
> restricted path.
> 
> I will highly appreciate your feedback.
> 
> 
> -- 
> Milan Budhathoki
> 
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-- 

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