 |
 |
Shaun, That was a really interesting read! Nice work and good writing! I haven't checked what you put on github, but it should prove useful to the OP at the conceptual if not the practical level. Chris On 11/11/2014 2:08 PM, Shaun Walbridge 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] <mailto:[log in to unmask]>> > Reply-To: a brody <[log in to unmask] <mailto:[log in to unmask]>> > Date: Tuesday, November 11, 2014 at 10:59 AM > To: "[log in to unmask] <mailto:[log in to unmask]>" > <[log in to unmask] <mailto:[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] > <mailto:[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] <mailto:[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] <mailto:[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] <mailto:[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] <mailto:[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. > -- Chris Duncan, Ph.D. President, GISmatters [log in to unmask] http://www.gismatters.com/ http://www.allterraindigital.com/ http://www.plugandplaymaps.com/ Tel: +1 413.549.2052 Fax: +1 413.658.0346 1 Tuckerman Ln Amherst, MA 01002 ------------------------------------------------------------------------- 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.