Country-coding from Lat/Long, using GeoPoly and/or R-Tree
(1) By ddevienne on 2020-04-21 11:27:10 [link]
Hi, I have tables of geometrical objects, with Latitude/Longitude coordinates, and we'd like to filter that data by **countries**. Given a set of polygons for countries, and [SQLite's GeoPoly](https://www.sqlite.org/geopoly.html) extension, it seems like this should be relatively easy. 1) Has anyone done that, and could share experience? 2) Where did you get the country boudaries? At which resolution? 3) Can the *lat/lon is within country X* be done efficiently using indexes? Perhaps an R-Tree index, for pre-filtering? 4) For *borderline* lat/lon, can nearby countries be returned as well? 5) How about perform? What should I watch out for? At this point, I'm just looking for community advise before diving into this myself, as I'm never used R-Tree and GeoPoly with SQLite. TIA, --DD
(2) By Gunter Hick (gunter_hick) on 2020-04-21 12:14:32 in reply to 1 [link]
Apart from some member states of the USA, countries do not tend to be rectangular. They may also consist of several disjunct areas aka overseas territories. The topological Definition of "inside" and "outside" is the parity of the count of intersections with the outline along an arbitrary path between the point of interest and a point whose inside/outside property is known. e.g. draw a straight line from the POI to the (center of) capital of the country. If the number of intersections is even (ideally zero for convex polygons), then the POI is "inside" the country. Intersecting at a tangent does not count.
(3) By Rob Willett (rwillett) on 2020-04-21 14:25:21 in reply to 2 [link]
(4) By Richard Hipp (drh) on 2020-04-21 14:48:59 in reply to 2 [link]
The "Geopoly" extension mentioned in the link of the original post supports non-rectangular regions. Each entry in Geopoly is a polygon. The extension supports primitives to: * Determine if polygons intersect * Determine if one polygon is entirely contained inside another * Determine if a point is inside or outside a polygon * Find the area of a polygon * Perform linear transformations (flipping, stretching, rotating) on polygons. * Render polygons as SVG. The inputs are GeoJSON descriptions of polygons. You can search online to find lots of maps in the GeoJSON format, which can be directly imported into a Geopoly index. An SQL script that demonstrates Geopoly's capabilities is in the source tree in the [ext/rtree/visual01.txt] file. Run this file as follows: ~~~~ sqlite3 <visual01.txt >geopoly-demo.html open geopoly-demo.html ~~~~ For your viewing convenience, I have uploaded the output to <https://sqlite.org/tmp/geopoly-demo.html>. : https://sqlite.org/src/file/ext/rtree/visual01.txt
(5) By Keith Medcalf (kmedcalf) on 2020-04-21 15:04:29 in reply to 1 [link]
Geopoly works quite well. I have implemented (among others) a system which will find the approporate IANA Timezone for a given lat/long -- it works quite well, with the following caveats: - there is a standard for polygon objects -- nothing follows it, especially things that claim they do - the SQLite3 implementation does not "support" holes - it is possible for one country to be entirely contained within another and you will have to figure out how to resolve a point being "inside" two "countries" at once - yes, you can find "near" countries simply by making you location "point" into a circle and getting the list of overlapping polygons - it is very fast but only uses single precision floating point so the resolution if you use native spherical coordinates (ie, lat/long) is about on par with uncorrected orbital pseudorange systems
(6) By ddevienne on 2020-04-21 15:49:47 in reply to 4 [link]
Thanks Richard. Very nice demo too. Upload for online access doubly nice too. BTW, your demo made me think about hover and click support on SVG elements, when inside the HTML page, and reading [this nice article](https://css-tricks.com/svg-map-rollovers/) (among others), it seems possible, but perhaps more control on `id` and/or `class` elements may be necessary for the SVG's `<polyline>` elements generated by the SQL function. I'll cross that bridge when I actually look into all that.
(7) By ddevienne on 2020-04-21 15:57:48 in reply to 5 [link]
Thanks Keith. All very good points. If you don't mind, where did you get your timezone polygons from? And what did you have to fix to get proper GeoJSON polys from those sources? One good source of data seems to be [Natural Earth Vector on GitHub](https://github.com/nvkelso/natural-earth-vector/tree/master/110m_physical), but the data comes in ShapeFile (.shp, .shx) and .DBF and .CPG formats, not something GeoPoly can use as-is :(
(8) By ddevienne on 2020-04-21 15:59:21 in reply to 2 [link]
Thanks Gunter. As I alluded to, and Richard confirmed, GeoPoly does these things. So this was a little too low-level an answer :). Appreciated nonetheless.
(9) By Keith Medcalf (kmedcalf) on 2020-04-21 16:14:05 in reply to 7 [link]
I got the data from GitHub. There is a lot of polygon files wafting around the Internet. Many of them claim varying levels of GeoJSON compliance. https://github.com/evansiroky/timezone-boundary-builder/releases is where I found the geojson files I used. The geojson files contained multiple polygons and sometimes multiple holes so they required a bit of pre-processing.
(10) By David Raymond (dvdraymond) on 2020-04-21 16:22:28 in reply to 1
As others have stated, country borders can be "fun". If I'm remembering R-Tree basically uses bounding boxes to help narrow down the candidates for you to do the more detailed check on. Consider for example that the Aleutian Islands in Alaska cross the 180th meridian. So the bounding box for the US is basically 360 degrees wide. So any point in the latitude range from the south of Hawaii to the north of Alaska will have the US show up as one of the possible countries to do a detailed check on. Depending on how detailed you need it you've also got multi-polys, holes, enclaves, whether the border ends at the ocean shore vs X nautical miles offshore, etc. And Lord help you if you're trying to do business in the countries with a border dispute. Country X will be rather harsh to you if you try to sell a product in their country that says area A is actually in country Y instead of being in country X like they claim it is. And likewise with country Y if you claim that A is in country X. But at least most of those are problems for the people making the map you're using, and not so much for you.
(11.1) Originally by Peter Johnson (missinglink) with edits by Richard Hipp (drh) on 2020-04-22 13:15:43 from 11.0 in reply to 7 [link]
We publish a huge amount of openly-licensed and free-of-charge geographic data here <https://geocode.earth/data/whosonfirst> Select the country you're after or 'XY' for things above the country-level such as Timezones, Oceans, Continents etc. If you click the 'name_compressed' link under 'SQLite Databases' you'll get a compressed SQLite database, in here you'll find an 'spr' table with a summary of the place and a 'geojson' table containing the geometries and metadata. The source of the data is the https://whosonfirst.org project which is managed by the same person as the Natural Earth project you linked.