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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: