I’ve done some work on global modeling of ship movement, and can confirm this is a tricky problem. The ultimate solution that I ended up using was a weighted graph structure to connect adjacent ‘cells’ of the ocean. This has the advantage of not requiring arbitrary new calculations for any new pair of points, and can vary in resolution. The edges are then weighted by their geodesic distance, so that cost calculations are ‘true’ and not fixed to any single projection. Some details here.

cheers,

Shaun

-- 

Shaun Walbridge | GIS Engineer

From: a brody <[log in to unmask]>
Reply-To: a brody <[log in to unmask]>
Date: Tuesday, November 11, 2014 at 10:59 AM
To: "[log in to unmask]" <[log in to unmask]>

Subject: Re: Finding Distance between points over water

FYI,

Mercator projection is a very poor measure of distances.  Its advantages are that the latitude and longitude lines come at right angles, make it easy for people plotting their routes, and determining the distance travelled in latitude.  Longitude varies though based on latitude. The best distance measurement for shortest distance is great circle routes when you can get it, but the problem remains many of those routes run into the arctic regions and iceburgs.  So the best compromise may vary by season for the hemisphere travelled.

It really depends on your end users needs.

Sincerely,

Abraham Brody
On Nov 11, 2014, at 12:14 PM, Don Cooke <[log in to unmask]> wrote:

A friend at GDT suggested a projection-free way to calculate distances between lat/long pairs:  calculate the spherical angle between the two points relative to the center of the earth, then convert the angle to a distance using an assumed radius of the earth.  I guess you need to assume a spherical earth….

 

Interesting way to think about it.  We do tend to get the rope wound around the axle given our comfort with projections.

 

****** Don

From: Northeast Arc Users Group [mailto:[log in to unmask]] On Behalf Of Andy Anderson
Sent: Tuesday, November 11, 2014 8:29 AM
To: [log in to unmask]
Subject: Re: Finding Distance between points over water

 

No single projection will work, you must use different ones over subglobal distances, which prevents a single calculation. But one approach would be to use the plate carrée projection (your attached example) to generate a rough route, then choose an appropriate projection over each leg, e.g. two-point equidistant, and then project to a new raster and redetermine the legs and then calculate the distance. Still won’t be the best possible distance because you’re fixing points, but it should be a better estimate.

 

For the resolution you are using (~70 Km/pixel) I don’t think a land buffer is necessary.

 

— Andy

 

On Nov 11, 2014, at 10:56 AM, Milan Budhathoki <[log in to unmask]> wrote:

Hi Chris and Andy,

 

Thank you for your suggestions. I was given just Lat/Long of ship's starting port and destination port in .CSV. Beside these I don't I have any additional information. Yes, prevailing winds and ocean currents will play a role in ships actual path but I think my collaborator is looking for the shortest distance just over water regardless of any other assumption that might affect on actual route. I haven't research at published route documents but this is worth trying. Projection is another issue since dataset has thousands of ports around the globe. I am thinking of  the "Merrcator Projection ?". I did a quick cost-path analysis for one of the ports pair. Due to lack of information to make a cost-raster ( I used just land and water mask as a cost raster) path tends to grip through land as below;

 

<image84 Nov. 11.jpg>
​

 

 

​

Here voyages tends to pass very closely to land (mostly touches land mask). I am thinking to buffer land mask outward with X miles so that ships path will be X miles way from land. Also I am considering to bring ocean depth as another cost-raster if data is publicly available. Eventually, I am write a python script or run model builder once I figure out better way.  

 

I would like to hear more if anyone has any thoughts ! 

 

Thank you ! 

 

 

 

 

On Mon, Nov 10, 2014 at 11:38 AM, Andy Anderson <[log in to unmask]> wrote:

To remove the tedium, write a Python script.

 

Remember, though, that distance calculations depend on the projection you use. If you want the cost-path from, say, New York to Sydney, there is no single projection that will give you an accurate measure.

 

A better approach might be to determine the standard shipping lanes, calculate the distances (if you can’t find them in a table), and piece together routes.

 

More generally, you could set up an iterative algorithm to calculate distances using spheroid-based angular calculations with restrictions based on open water (e.g. at 40° north latitude, longitude will be restricted to roughly –74° to –9° and 128° to 140° and 142° to -124°).

 

— Andy

 

On Nov 10, 2014, at 9:13 AM, Milan Budhathoki <[log in to unmask]> 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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--

Milan Budhathoki

 

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