parse-gml-polygon
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Convert a GML Polygon into GeoJSON.
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# parse-gml-polygon
**Convert a [GML](https://en.wikipedia.org/wiki/Geography_Markup_Language) `Polygon` into a [GeoJSON](http://geojson.org) geometry.** Accepts the format of [`xml-reader`](https://www.npmjs.com/package/xml-reader) (with the `preserveMarkup` flag) as input. Works with a subset of [GML 3.3](https://portal.opengeospatial.org/files/?artifact_id=46568).
**Parsing GML is a nightmare. This module tries to parse most of the mentioned ways to encode a polygon.** I don't intend to cover all of them though. To quote [the wonderful *GML madness* article by Even Rouault](http://erouault.blogspot.de/2014/04/gml-madness.html):
> But, you may have noticed that the child of a `CompositeCurve` is a `curveMember`, which is also the parent of the `CompositeCurve`. So we may put a `CompositeCurve` inside a `CompositeCurve`.
> […] or maybe you prefer to use `gml:surfaceMembers` (with a final `s`) instead of a `gml:surfaceMember` […]
> To conclude, we should mention that the authors of the GML specification have admitted that encoding polygons was a bit too complicated.
[](https://www.npmjs.com/package/parse-gml-polygon)
[](https://travis-ci.org/derhuerst/parse-gml-polygon)
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## Installing
```shell
npm install parse-gml-polygon
```
## Example
```js
const h = require('hyper-xml')
const parse = require('parse-gml-polygon')
const el = h('gml:Polygon', {'gml:id': 'some-id'}, [
h('gml:exterior', [
// triangle of 0|0 0|3 3|3
h('gml:LinearRing', [
h('gml:posList', ['0 0 0 3 3 3 0 0'])
])
]),
h('gml:interior', [
// triangle of 1|1 1|2 2|2
h('gml:LinearRing', [
h('gml:posList', ['1 1 1 2 2 2 1 1'])
])
])
])
const geometry = parse(el)
console.log(geometry)
```
```js
{
type: 'Polygon',
coordinates: [
[ // exterior/outer shape
[0, 0],
[0, 3],
[3, 3],
[0, 0]
],
[ // interior/inner shape
[1, 1],
[1, 2],
[2, 2],
[1, 1]
]
]
}
```
## Usage
This library consumes a tree structure corresponding to XML. [`xml-reader`](https://www.npmjs.com/package/xml-reader) parses XML into these structures. With [`hyper-xml`](https://npmjs.com/package/hyper-xml), you can create them manually.
Look at these code examples to understand how to use `parse-gml-polygon`:
- from an XML/GML string: [`examples/xml-string.js`](examples/xml-string.js)
- from an XML/GML file: [`examples/xml-file.js`](examples/xml-file.js)
- creating a tree manually: [`examples/hyper-xml.js`](examples/hyper-xml.js)
## API
```js
parseGmlPolygon(tree, { transformCoords: noTransform, stride: 2}) => GeoJSON
```
Second argument is a map of options:
- You may optionally pass in a `transformCoords` function, e.g. to translate them into WGS84. The default transform is `(x, y) => [x, y]`.
- `stride` specifies the default number of values that each point in the polygon has. A `stride` of `3` would correspond to a polygon in a 3D coordinate system.
Please note that this value can be overridden by the `srsDimension` attribute.
Default value of `stride` is `2`.
## Unsupported encodings
- `<gml:pointProperty xlink:href="#some-point-id"/>`
- `gml:coordinates`, which is deprecated
- see `todo`s in [the tests](test.js)
## Contributing
If you have a question or have difficulties using `parse-gml-polygon`, please double-check your code and setup first. If you think you have found a bug or want to propose a feature, refer to [the issues page](https://github.com/derhuerst/parse-gml-polygon/issues).