Given a lat/long point, determine if it's land or water?

Question

I have a PostGIS-enabled PostgreSQL 9.1 installation with the 10M physical dataset from Natural Earth imported. It's been converted to 900913, and in QGIS I can visualize the data. So the plumbing's all set.

I have a very simple task: I'd like a SQL query to determine if a given point is land or water. Here's a very simple use case: user mouse-downs on a slippy map, the server takes that point and replies "that's land" or "that's not land." I feel that this is surely accomplished with the ne_10m_land layer (with enough accuracy for my purposes).

I'm a newbie to GIS, so while I've tried to do my homework I feel like I must be missing something really obvious to accomplish this...

Answer 1

I am asuming that your slippy map will be a JavaScript Application that runs withing a Browser.

JavaScript cannot directly access your Database. It will require an intermediary Service, that will make queries on your database, and return the result.

Your service could take in the x & y Parameters of the point the user has clicked, and then do the following query:

select (count(*) >0) as isLand from "10m_land" where 
ST_Intersects(ST_SetSRID(ST_MakePoint(:x,:y), 3857), the_geom);

I am assuming that your map, and hence the point that user has clicked, is in Web Mercator Projection (i.e. with wkid=3857). The query will try to find the number of features that intersect with the given point, and if that number is greater than 0, will return true, otherwise return false.

Answer 2

There is a free web API that solves exactly this problem called onwater.io. Given a latitude and longitude it will accurately return true or false via a get request.

Example on water: https://api.onwater.io/23.92323,-66.3

{
  lat: 23.92323,
  lon: -66.3,
  water: true
}

Example on land: https://api.onwater.io/42.35,-71.1

{
  lat: 42.35,
  lon: -71.1,
  water: false
}

Answer 3

Similar to OnWater (which shut down) there an API service called IsItWater.com which will let you check:

Request

curl 'https://isitwater-com.p.rapidapi.com/?latitude=41.9029192&longitude=-70.2652276&rapidapi-key=YOUR-X-RAPIDAPI-KEY'

Response

{
    "water": true,
    "latitude": 41.9029192,
    "longitude": -70.2652276
}