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Without knowing specifically, I feel like there has got to be a way to get satisfactory results using a small number of equidistant projections. Are there a couple of ocean-centric equidistant projections that you could use, splitting the routes into trans-Atlantic and Trans-Pacific routes, and handling each with its own projection? I am picturing a pin-cushion effect on the parallels to account for great-circle distances approaching the poles... Maybe... Jeff On Tue, Nov 11, 2014 at 2:08 PM, Shaun Walbridge <[log in to unmask]> wrote: > 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 <https://4326.us/thesis/>. > > 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] > <[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 > > > > ------------------------------------------------------------------------- > This list (NEARC-L) is an unmoderated discussion list for all NEARC Users. > > If you no longer wish to receive e-mail from this list, you can remove > yourself by going to http://listserv.uconn.edu/nearc-l.html. > > > > ------------------------------------------------------------------------- > This list (NEARC-L) is an unmoderated discussion list for all NEARC Users. > > If you no longer wish to receive e-mail from this list, you can remove > yourself by going to http://listserv.uconn.edu/nearc-l.html. > > > > > > -- > > Milan Budhathoki > > > > ------------------------------------------------------------------------- > This list (NEARC-L) is an unmoderated discussion list for all NEARC Users. > > If you no longer wish to receive e-mail from this list, you can remove > yourself by going to http://listserv.uconn.edu/nearc-l.html. > ------------------------------------------------------------------------- This list (NEARC-L) is an unmoderated discussion list for all NEARC Users. 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