README.md

Layer Stats

This page describes how to generate and analyze layer stats data to find ways to optimize tile size.

Generating Layer Stats

Run planetiler with --output-layerstats to generate an extra <output>.layerstats.tsv.gz file with a row for each layer in each tile that can be used to analyze tile sizes. You can also get stats for an existing archive by running:

java -jar planetiler.jar stats --input=<path to mbtiles or pmtiles file> --output=layerstats.tsv.gz

The output is a gzipped tsv with a row per layer on each tile and the following columns:

column	description
z	tile zoom
x	tile x
y	tile y
hilbert	tile hilbert ID (defines pmtiles order)
archived_tile_bytes	stored tile size (usually gzipped)
layer	layer name
layer_bytes	encoded size of this layer on this tile
layer_features	number of features in this layer
layer_geometries	number of geometries in features in this layer, including inside multipoint/multipolygons/multilinestring features
layer_attr_bytes	encoded size of the attribute key/value pairs in this layer
layer_attr_keys	number of distinct attribute keys in this layer on this tile
layer_attr_values	number of distinct attribute values in this layer on this tile

Analyzing Layer Stats

Load a layer stats file in duckdb:

CREATE TABLE layerstats AS SELECT * FROM 'output.pmtiles.layerstats.tsv.gz';

Then get the biggest layers:

SELECT * FROM layerstats ORDER BY layer_bytes DESC LIMIT 2;

z	x	y	hilbert	archived_tile_bytes	layer	layer_bytes	layer_features	layer_geometries	layer_attr_bytes	layer_attr_keys	layer_attr_values
14	13722	7013	305278258	1260526	housenumber	2412589	108390	108390	30764	1	3021
14	13723	7014	305278256	1059752	housenumber	1850041	83038	83038	26022	1	2542

To get a table of biggest layers by zoom:

PIVOT (
  SELECT z, layer, (max(layer_bytes)/1000)::int size FROM layerstats GROUP BY z, layer ORDER BY z ASC
) ON printf('%2d', z) USING sum(size);
-- duckdb sorts columns lexicographically, so left-pad the zoom so 2 comes before 10

layer	0	1	2	3	4	5	6	7	8	9	10	11	12	13	14
boundary	10	75	85	53	44	25	18	15	15	29	24	18	32	18	10
landcover	2	1	8	5	3	31	18	584	599	435	294	175	166	111	334
place	116	314	833	830	525	270	165	80	51	54	63	70	50	122	221
water	8	4	11	9	15	13	89	114	126	109	133	94	167	116	91
water_name	7	19	25	15	11	6	6	4	3	6	5	4	4	4	29
waterway				1	4	2	18	13	10	28	20	16	60	66	73
park					54	135	89	76	72	82	90	56	48	19	50
landuse					3	2	33	67	95	107	177	132	66	313	109
transportation					384	425	259	240	287	284	165	95	313	187	133
transportation_name							32	20	18	13	30	18	65	59	169
mountain_peak								13	13	12	15	12	12	317	235
aerodrome_label									5	4	5	4	4	4	4
aeroway											16	26	35	31	18
poi													35	18	811
building														94	1761
housenumber															2412

To get biggest tiles:

CREATE TABLE tilestats AS SELECT
z, x, y,
any_value(archived_tile_bytes) gzipped,
sum(layer_bytes) raw
FROM layerstats GROUP BY z, x, y;

SELECT
z, x, y,
format_bytes(gzipped::int) gzipped,
format_bytes(raw::int) raw,
FROM tilestats ORDER BY gzipped DESC LIMIT 2;

NOTE: this group by uses a lot of memory so you need to be running in file-backed mode duckdb analysis.duckdb (not in-memory mode)

z	x	y	gzipped	raw
13	2286	3211	9KB	12KB
13	2340	2961	9KB	12KB

To make it easier to look at these tiles on a map, you can define following macros that convert z/x/y coordinates to lat/lons:

CREATE MACRO lon(z, x) AS (x/2**z) * 360 - 180;
CREATE MACRO lat_n(z, y) AS pi() - 2 * pi() * y/2**z;
CREATE MACRO lat(z, y) AS degrees(atan(0.5*(exp(lat_n(z, y)) - exp(-lat_n(z, y)))));
CREATE MACRO debug_url(z, x, y) as concat(
  'https://protomaps.github.io/PMTiles/#map=',
  z + 0.5, '/',
  round(lat(z, y + 0.5), 5), '/',
  round(lon(z, x + 0.5), 5)
);

SELECT z, x, y, debug_url(z, x, y), layer, format_bytes(layer_bytes) size
FROM layerstats ORDER BY layer_bytes DESC LIMIT 2;

z	x	y	debug_url(z, x, y)	layer	size
14	13722	7013	https://protomaps.github.io/PMTiles/#map=14.5/25.05575/121.51978	housenumber	2.4MB
14	13723	7014	https://protomaps.github.io/PMTiles/#map=14.5/25.03584/121.54175	housenumber	1.8MB

Drag and drop your pmtiles archive to the pmtiles debugger to see the large tiles on a map. You can also switch to the "inspect" tab to inspect an individual tile.

Computing Weighted Average Tile Sizes

If you compute a straight average tile size, it will be dominated by ocean tiles that no one looks at. You can compute a weighted average based on actual usage by joining with a z, x, y, loads tile source. For convenience, top_osm_tiles.tsv.gz has the top 1 million tiles from 90 days of OSM tile logs from summer 2023.

You can load these sample weights using duckdb's httpfs module:

INSTALL httpfs;
CREATE TABLE weights AS SELECT z, x, y, loads FROM 'https://raw.githubusercontent.com/onthegomap/planetiler/main/layerstats/top_osm_tiles.tsv.gz';

Then compute the weighted average tile size:

SELECT
format_bytes((sum(gzipped * loads) / sum(loads))::int) gzipped_avg,
format_bytes((sum(raw * loads) / sum(loads))::int) raw_avg,
FROM tilestats JOIN weights USING (z, x, y);

gzipped_avg	raw_avg
81KB	132KB

If you are working with an extract, then the low-zoom tiles will dominate, so you can make the weighted average respect the per-zoom weights that appear globally:

WITH zoom_weights AS (
  SELECT z, sum(loads) loads FROM weights GROUP BY z
),
zoom_avgs AS (
  SELECT
  z,
  sum(gzipped * loads) / sum(loads) gzipped,
  sum(raw * loads) / sum(loads) raw,
  FROM tilestats JOIN weights USING (z, x, y)
  GROUP BY z
)
SELECT
format_bytes((sum(gzipped * loads) / sum(loads))::int) gzipped_avg,
format_bytes((sum(raw * loads) / sum(loads))::int) raw_avg,
FROM zoom_avgs JOIN zoom_weights USING (z);