If you ensure winding order, calculating the area of a polygon with holes is as simple as adding the areas of all rings. The classic Chamberlain & Duquette algorithm for calculating the area of a polygon on a sphere has the nice property that counterclockwise-wound polygons have positive area and clockwise yield negative.Why care? There are roughly two practical reasons: The exterior ring should be counterclockwise.Polygon ring order is undefined in GeoJSON, but there’s a useful default to acquire: the right hand rule. The direction of LineString often reflects the direction of something in real life: a GPS trace will go in the direction of movement, or a street in the direction of allowed traffic flows. LineString and Polygon geometries contain coordinates in an order: lines go in a certain direction, and polygon rings do too. UPDATE: RFC 7946 GeoJSON now recommends right-hand rule winding order This is really nifty for implementations: they don’t need to guess about what kind of GeoJSON object they’re looking at - they just read the “type” property. GeoJSON objects are Objects, not Arrays or primitives.You might ask “why not just permit an array of GeoJSON objects”? FeatureCollections as objects makes a lot of sense in terms of the commonality between different GeoJSON types. As the name suggests, the array needs to contain Feature objects only - no raw geometries. įeatureCollection is not much more than an object that has "type": "FeatureCollection" and then an array of Feature objects under the key "features". With a single position, we can make the simplest geometry: the point. All simple geometries in GeoJSON consist of a type and a collection of coordinates. This wasn’t well supported in GeoJSON tools, and is forbidden by the new specification. Here’s a handy chart of what uses which ordering.īefore the current specification was released, GeoJSON allowed the storage of more than 3 coordinates per position, and sometimes people would use that to store time, heart rate, and so on. applications have tended to use latitude, longitude order.data formats usually use longitude, latitude order.longitude, latitude matches the X, Y order of math.Long hours have been wasted discussing which is better, but for this discussion, I’ll summarize as such: Historically, the order of coordinates is usually “latitude, longitude”, and many people will assume that this is the case universally. GeoJSON describes an order for coordinates: they should go, in order: PositionĪ position is an array of coordinates in order: this is the smallest unit that we can really consider ‘a place’ since it can represent a point on earth. As cool as 8° 10’ 23” looks, it’s just not a very good way to tell numbers to computers. Unlike geographic data for human consumption, data formats never use non-base-10 encodings like sexagesimal. Time is a dimension but usually isn’t represented in a coordinate because it’s too complex to fit in a number.Ĭoordinates in GeoJSON are formatted like numbers in JSON: in a simple decimal format. Sometimes there’s also a coordinate for elevation. This is a single number representing a single dimension: typically the dimensions are longitude and latitude. The most basic element of geographic data is the coordinate. This should be read along with the GeoJSON spec itself, which is authoritative and, for a formal specification of a format, pretty readable. Understanding these concepts will help you understand geospatial data in general, too: the basic concepts behind GeoJSON have been a part of geo since the very beginning. Let’s look at GeoJSON in a little more depth, from the ground up. Psst: working with maps? Try out Placemark, my map-making tool with great support for GeoJSON, KML, and much more!
0 kommentar(er)
