Add SpatialRef::to_projjson()

CHANGES.md

@@ -1,6 +1,10 @@
 # Changes
 
 ## Unreleased
+- Added `SpatialRef::to_projjson`
+
+  - <https://github.com/georust/gdal/pull/389>
+
 - Added `Geometry::length`
 
   - <https://github.com/georust/gdal/pull/384>

src/spatial_ref/srs.rs

@@ -506,6 +506,22 @@ impl SpatialRef {
         res
     }
 
+    pub fn to_projjson(&self) -> Result<String> {
+        let mut c_projjsonstr = ptr::null_mut();
+        let options = ptr::null();
+        let rv = unsafe { gdal_sys::OSRExportToPROJJSON(self.0, &mut c_projjsonstr, options) };
+        let res = if rv != OGRErr::OGRERR_NONE {
+            Err(GdalError::OgrError {
+                err: rv,
+                method_name: "OSRExportToPROJJSON",
+            })
+        } else {
+            Ok(_string(c_projjsonstr))
+        };
+        unsafe { gdal_sys::VSIFree(c_projjsonstr.cast::<std::ffi::c_void>()) };
+        res
+    }
+
     pub fn auth_name(&self) -> Result<String> {
         let c_ptr = unsafe { gdal_sys::OSRGetAuthorityName(self.0, ptr::null()) };
         if c_ptr.is_null() {

src/spatial_ref/tests.rs

@@ -43,6 +43,17 @@ fn from_epsg_to_wkt_proj4() {
     assert_eq!("+proj=longlat +datum=WGS84 +no_defs", proj4string.trim());
 }
 
+#[test]
+fn from_epsg_to_projjson() {
+    let spatial_ref = SpatialRef::from_epsg(4326).unwrap();
+    let projjson = spatial_ref.to_projjson().unwrap();
+    // Testing for exact string equality would be too strict, since the order of keys in JSON is
+    // unspecified. Ideally, we'd parse the JSON and then compare the values, but adding a JSON
+    // parser as a dependency just for this one test would be overkill. Thus, we do only a quick
+    // sanity check.
+    assert!(projjson.contains("\"World Geodetic System 1984 (G1674)\""));
+}
+
 #[test]
 fn from_esri_to_proj4() {
     let spatial_ref = SpatialRef::from_esri("GEOGCS[\"GCS_WGS_1984\",DATUM[\"D_WGS_1984\",SPHEROID[\"WGS_1984\",6378137,298.257223563]],PRIMEM[\"Greenwich\",0],UNIT[\"Degree\",0.017453292519943295]]").unwrap();