Timeline for Calculating length of Appalachian Trail
Current License: CC BY-SA 3.0
20 events
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| Jul 20, 2017 at 20:17 | comment | added | ziggy | @Angelo.Hannes good point, didnt think of that. just trying to brainstorm | |
| Jul 20, 2017 at 20:16 | comment | added | Angelo.Hannes | @ziggy wouldn't that leave out all the turns, since a geom data contains a multilinestring? | |
| Jul 20, 2017 at 19:43 | comment | added | ziggy | what about st_distance_spheroid? take the two end points of the linestring and compute the distance??...gis.stackexchange.com/questions/109562/… | |
| Jul 20, 2017 at 19:32 | comment | added | Angelo.Hannes | @ThingumaBob Thank you very much for your help, appreciate it! I wasn't expecting to get to the exact length the ATC uses. However, starting from the same dataset, arriving at a difference of 70 Miles is way to big. | |
| Jul 20, 2017 at 19:19 | comment | added | geozelot | well...assuming the SRIDs are correct, I suspect that there is not much more you can do. theoretically adding the 80 miles in elevation gain/loss gets you closer, but as I said, who knows how they measured the lenght. could be with different data, different projections or by car. I thought you could get closer with the right CRS, but apparently no. nevertheless: you played a lot with PostGIS, can't say you didn't learn ,) | |
| Jul 20, 2017 at 19:18 | answer | added | Inactivated Account | timeline score: 2 | |
| Jul 20, 2017 at 19:09 | comment | added | Angelo.Hannes | @ThingumaBob Ok, so I reimported the dataset and set the srid from 0 to 4269. Still no difference | |
| Jul 20, 2017 at 19:00 | comment | added | geozelot | sorry, yes, using updateGeometrySRID actually transforms the datas SRID from its current to the one given (both 4326 in your case, so nothing changed...). what you want is to make sure that you either imported your data with the correct one or, if none is set, set it to the right one. | |
| Jul 20, 2017 at 18:55 | comment | added | Angelo.Hannes |
@ThingumaBob So I updated my table with select updateGeometrySRID('at_centerline','geom',4269) and queried with select sum(st_length(st_transform(geom, 4326)::geography))/1000*0.621371 from at_centerline and still get 2122 miles
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| Jul 20, 2017 at 18:49 | comment | added | geozelot | that should be NAD83 (EPSG:4269). update your geom column to that and transform to WGS84 to use the geography type. | |
| Jul 20, 2017 at 18:40 | comment | added | Angelo.Hannes |
@ThingumaBob The projection is probably GEOGCS["GCS_North_American_1983",DATUM["D_North_American_1983",SPHEROID["GRS_1980",6378137.0,298.257222101]],PRIMEM["Greenwich",0.0],UNIT["Degree",0.0174532925199433]]
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| Jul 20, 2017 at 18:23 | comment | added | geozelot | I just read that the ATC also distributes its data in NAD83 (also lat/lon), which might make a little difference. Maybe check the metadata of the shapefiles. A cast to geography usually results in more precise/realistic measures, as it calculates the distance based on the spheroid. Depending on your PostGIS version, this defaults to WGS84. If your data is in NAD83, you'll need to transform to WGS84 first. But in general, I doubt that you will get exactly to the 2190 miles...who knows how they measured that lenght? | |
| Jul 20, 2017 at 18:06 | comment | added | Angelo.Hannes | @JGH that isn't the highest point. it is the "Approximate Gain/Loss in Elevation" | |
| Jul 20, 2017 at 18:04 | comment | added | JGH | up-down, up-down, up-down... that adds up much more than going straight from start to the highest point | |
| Jul 20, 2017 at 18:03 | comment | added | Angelo.Hannes | Because I didn't read this answer thoroughly and only copied the query: gis.stackexchange.com/questions/143436/… | |
| Jul 20, 2017 at 18:00 | comment | added | ziggy | the SRID of 2877 is a colorado state plane CS..why would you use that? | |
| Jul 20, 2017 at 17:49 | comment | added | Angelo.Hannes | There seems to be an elevation of around 88 Miles. Using Pythagorean Theorem I get 2124.4 miles considering elevation. This doesn't seem to be the error. | |
| Jul 20, 2017 at 17:33 | comment | added | JGH | You are measuring distances on flat ground. Maybe the 2190 miles is considering the slope? | |
| Jul 20, 2017 at 16:52 | review | First posts | |||
| Jul 20, 2017 at 16:57 | |||||
| Jul 20, 2017 at 16:52 | history | asked | Angelo.Hannes | CC BY-SA 3.0 |