Add Hilbert Curve feature sequencing as an option

README.md

@@ -425,7 +425,7 @@ the same layer, enclose them in an `all` expression so they will all be evaluate
  * `-as` or `--drop-densest-as-needed`: If a tile is too large, try to reduce it to under 500K by increasing the minimum spacing between features. The discovered spacing applies to the entire zoom level.
  * `-ad` or `--drop-fraction-as-needed`: Dynamically drop some fraction of features from each zoom level to keep large tiles under the 500K size limit. (This is like `-pd` but applies to the entire zoom level, not to each tile.)
  * `-an` or `--drop-smallest-as-needed`: Dynamically drop the smallest features (physically smallest: the shortest lines or the smallest polygons) from each zoom level to keep large tiles under the 500K size limit. This option will not work for point features.
- * `-aN` or `--coalesce-smallest-as-needed`: Dynamically combine the smallest features (physically smallest: the shortest lines or the smallest polygons) from each zoom level into other nearby features to keep large tiles under the 500K size limit. This option will not work for point features, and will probably not help very much with LineStrings. It is mostly intended for polygons, to maintain the full original area covered by polygons while still reducing the feature count somehow. The attributes of the small polygons are *not* preserved into the combined features, only their geometry. Furthermore, the polygons that nested polygons are coalesced to may not necessarily be the immediately enclosing features.
+ * `-aN` or `--coalesce-smallest-as-needed`: Dynamically combine the smallest features (physically smallest: the shortest lines or the smallest polygons) from each zoom level into other nearby features to keep large tiles under the 500K size limit. This option will not work for point features, and will probably not help very much with LineStrings. It is mostly intended for polygons, to maintain the full original area covered by polygons while still reducing the feature count somehow. The attributes of the small polygons are *not* preserved into the combined features (except through `--accumulate-attribute`), only their geometry. Furthermore, the polygons to which nested polygons are coalesced may not necessarily be the immediately enclosing features.
  * `-aD` or `--coalesce-densest-as-needed`: Dynamically combine the densest features from each zoom level into other nearby features to keep large tiles under the 500K size limit. (Again, mostly useful for polygons.)
  * `-aS` or `--coalesce-fraction-as-needed`: Dynamically combine a fraction of features from each zoom level into other nearby features to keep large tiles under the 500K size limit. (Again, mostly useful for polygons.)
  * `-pd` or `--force-feature-limit`: Dynamically drop some fraction of features from large tiles to keep them under the 500K size limit. It will probably look ugly at the tile boundaries. (This is like `-ad` but applies to each tile individually, not to the entire zoom level.) You probably don't want to use this.
@@ -458,9 +458,10 @@ the same layer, enclose them in an `all` expression so they will all be evaluate
 ### Reordering features within each tile
 
  * `-pi` or `--preserve-input-order`: Preserve the original input order of features as the drawing order instead of ordering geographically. (This is implemented as a restoration of the original order at the end, so that dot-dropping is still geographic, which means it also undoes `-ao`).
- * `-ao` or `--reorder`: Reorder features to put ones with the same attributes in sequence, to try to get them to coalesce. You probably want to use this if you use `--coalesce`.
- * `-ac` or `--coalesce`: Coalesce adjacent features that have the same attributes. This can be useful if you have lots of small polygons with identical attributes and you would like to merge them together.
+ * `-ac` or `--coalesce`: Coalesce consecutive features that have the same attributes. This can be useful if you have lots of small polygons with identical attributes and you would like to merge them together.
+ * `-ao` or `--reorder`: Reorder features to put ones with the same attributes in sequence (instead of ones that are approximately spatially adjacent), to try to get them to coalesce. You probably want to use this if you use `--coalesce`.
  * `-ar` or `--reverse`: Try reversing the directions of lines to make them coalesce and compress better. You probably don't want to use this.
+ * `-ah` or `--hilbert`: Put features in Hilbert Curve order instead of the usual Z-Order. This improves the odds that spatially adjacent features will be sequentially adjacent, and should improve density calculations and spatial coalescing. It should be the default eventually.
 
 ### Adding calculated attributes
 
@@ -469,7 +470,7 @@ the same layer, enclose them in an `all` expression so they will all be evaluate
 
 ### Trying to correct bad source geometry
 
- * `-aw` or `--detect-longitude-wraparound`: Detect when adjacent points within a feature jump to the other side of the world, and try to fix the geometry.
+ * `-aw` or `--detect-longitude-wraparound`: Detect when consecutive points within a feature jump to the other side of the world, and try to fix the geometry.
  * `-pw` or `--use-source-polygon-winding`: Instead of respecting GeoJSON polygon ring order, use the original polygon winding in the source data to distinguish inner (clockwise) and outer (counterclockwise) polygon rings.
  * `-pW` or `--reverse-source-polygon-winding`: Instead of respecting GeoJSON polygon ring order, use the opposite of the original polygon winding in the source data to distinguish inner (counterclockwise) and outer (clockwise) polygon rings.
  * `--clip-bounding-box=`*minlon*`,`*minlat*`,`*maxlon*`,`*maxlat*: Clip all features to the specified bounding box.