Feature-specific zoom calculations (#163)

* start calculating latitude and longitude deltas between source and vector tile

* start calculating more data-specific zoom levels

* set smallest max zoom for point files

* constants

* stick with z10 for now

* update expected zooms

* update sm version

* update appveyor node versions

* pin node versions

appveyor.yml

@@ -2,7 +2,8 @@ os: Visual Studio 2015
 
 environment:
   matrix:
-    - nodejs_version: 0.10.40
+    - nodejs_version: 4.4.1
+    - nodejs_version: 6.5.0
 
 platform:
   - x64

lib/csv.js

@@ -69,5 +69,5 @@ Csv.prototype.getLayers = function(callback) {
 };
 
 Csv.prototype.getZooms = function(callback) {
-  utils.zoomsBySize(this.filepath, this.extent, callback);
+  utils.zoomsBySize(this.filepath, this.extent, this.datasource, callback);
 };

lib/geojson.js

@@ -75,5 +75,5 @@ GeoJSON.prototype.getLayers = function(callback) {
 };
 
 GeoJSON.prototype.getZooms = function(callback) {
-  utils.zoomsBySize(this.filepath, this.extent, callback);
+  utils.zoomsBySize(this.filepath, this.extent, this.datasource, callback);
 };

lib/ogr.js

@@ -67,6 +67,8 @@ Ogr.prototype.getZooms = function(callback) {
 
   this.getExtent(function(err, extent) {
     if (err) return callback(err);
+
+    // no datasource object like mapnik, so don't pass it in
     utils.zoomsBySize(filepath, extent, callback);
   });
 };

lib/shape.js

@@ -129,9 +129,10 @@ Shape.prototype.getLayers = function(callback) {
 
 Shape.prototype.getZooms = function(callback) {
   var filepath = this.filepath;
+  var ds = this.datasource;
 
   this.getExtent(function(err, extent) {
     if (err) return callback(err);
-    utils.zoomsBySize(filepath, extent, callback);
+    utils.zoomsBySize(filepath, extent, ds, callback);
   });
 };

lib/topojson.js

@@ -64,5 +64,5 @@ TopoJSON.prototype.getLayers = function(callback) {
 };
 
 TopoJSON.prototype.getZooms = function(callback) {
-  utils.zoomsBySize(this.filepath, this.extent, callback);
+  utils.zoomsBySize(this.filepath, this.extent, this.datasource, callback);
 };

lib/utils.js

@@ -3,7 +3,15 @@ var invalid = require('./invalid');
 var SphericalMercator = require('@mapbox/sphericalmercator');
 var sm = new SphericalMercator();
 
-module.exports.zoomsBySize = function(filepath, extent, callback) {
+var SMALLEST_MAX_ZOOM_FILE_SIZE = 1000000; // 1mb
+var DEFAULT_SMALLEST_MAX_ZOOM = 6;
+
+module.exports.zoomsBySize = function(filepath, extent, datasource, callback) {
+
+  // currently the ogr plugin doesn't pass in a datasource,
+  // so we need to redefine the callback
+  if (typeof datasource === 'function') callback = datasource;
+
   var maxSize = 500 * 1024;
   var max = 22;
   var min;
@@ -18,6 +26,9 @@ module.exports.zoomsBySize = function(filepath, extent, callback) {
     var tiles;
     var avg;
 
+    // set a "smallest max zoom" for different data types
+    var smallestMaxZoom = stats.size < SMALLEST_MAX_ZOOM_FILE_SIZE ? dataTypeMaxZoom(datasource) : DEFAULT_SMALLEST_MAX_ZOOM;
+
     for (z = max; z >= 0; z--) {
       bounds = sm.xyz(extent, z, false, 4326);
       x = (bounds.maxX - bounds.minX) + 1;
@@ -35,18 +46,45 @@ module.exports.zoomsBySize = function(filepath, extent, callback) {
       avg = stats.size / tiles;
 
       if (avg < 1000) max = z;
-
       if (avg > maxSize) {
         min = z;
-        return callback(null, min, Math.max(max, 6));
+        return callback(null, min, Math.max(max, smallestMaxZoom));
       } else if (tiles === 1 || z === 0) {
         min = 0;
-        return callback(null, min, Math.max(max, 6));
+        return callback(null, min, Math.max(max, smallestMaxZoom));
       }
     }
   });
 };
 
+/**
+ * Calculates the smallest max zoom a datasource should be tiled to. This is used for small datasources
+ * where precision can be lost due to large tile extents. https://github.com/mapbox/mapnik-omnivore/issues/151
+ *
+ * @param {object} datasource A Node Mapnik datasource object
+ * @returns {number} zoom The smallest max zoom to use for the datasource
+ */
+module.exports.dataTypeMaxZoom = dataTypeMaxZoom;
+var dataTypeMaxZoom = function(ds) {
+
+  // try describing the datasource (this catches the ogr hole)
+  var info;
+  try {
+    info = ds.describe();
+  } catch (err) {
+    return DEFAULT_SMALLEST_MAX_ZOOM;
+  }
+
+  // for point features, we are able to keep within ~50cm (1.5ft) precision at z11
+  // https://github.com/mapbox/unpacker/issues/1248#issuecomment-256963081
+  if (info.geometry_type === 'point') {
+    return 10;
+  }
+
+  // return 6 for all other data types for now
+  return DEFAULT_SMALLEST_MAX_ZOOM;
+};
+
 module.exports.convertToMeters = function(pixelSize, unit) {
   var circumference = 40075000;
   var conversions = {

package.json

@@ -9,7 +9,7 @@
     "geodata"
   ],
   "dependencies": {
-    "@mapbox/sphericalmercator": "~1.0.2",
+    "@mapbox/sphericalmercator": "~1.0.5",
     "@mapbox/mapbox-file-sniff": "~1.0.0",
     "srs": "~1.2.0",
     "mapnik": "~3.5.14",