GPX slippy map

This renders GPX files on-the-fly to a transparent slippy-map layer, allowing you to browse tracklogs in more detail than the existing preview image.

Front page is at:

http://dev.openstreetmap.org/~ojw/gpx/list.php

Note thas to find the .gpx ID, you have to go to your own gpx page and try to download it. Something like 1234567.gpx will be proposed, the first 7 numbers are the ID.

Or browse to a URL of the form:

http://dev.openstreetmap.org/~ojw/gpx/?gpx=[the GPX trace ID]

e.g.

http://dev.openstreetmap.org/~ojw/gpx/?gpx=113257

Contents 1 Overview 2 Source code 3 Benchmarks 4 Optimisations done 5 Optimisations to do 6 How you can help 7 Binary file format 8 Testing

Overview

Source code

Download from:

http://svn.openstreetmap.org/applications/rendering/gpx_slippy_map/

revision history is viewable at:

http://trac.openstreetmap.org/browser/applications/rendering/gpx_slippy_map

Benchmarks

For a 9999-point tracklog:

• Render tile containing data: 140 ms
• Download and process new tracklog: 800ms
• Render tile without data: 3 ms

Optimisations done

• check if bounding box is completely outside the tile, and if so don't draw anything
• precalculate mercator projections when storing tracklog, rather than when drawing tile
• save some space by storing points as integer not floating point

Optimisations to do

• Support for gziped gpx files i.e. 2009_03_29_1600.gpx.gz

please add any suggestions here

How you can help

• If someone could setup a cache in front of the tileserver that would be great (i.e. subsequent HTTP requests for the same tile should come from your disk not from dev server)

GPX tiles never expire if that helps your caching policy (because a given GPX-id/x/y/z will always return the same image unless the rendering software changes)

Binary file format

# +---------+--------+------+--------------------------+
# | size    | type   | name | description              |
# +---------+--------+------+--------------------------+
# | 4 bytes | int    | n    | number of trackpoints    |
# | 8 bytes | double | xmin | bounding box (W)         |
# | 8 bytes | double | xmax | bounding box (E)         |
# | 8 bytes | double | ymin | bounding box (S)         |
# | 8 bytes | double | ymax | bounding box (N)         |
# +---------+--------+------+--------------------------+
#   then n pairs of:
# +---------+--------+------+--------------------------+
# | 4 bytes | int    | x    | coded position           |
# | 4 bytes | int    | y    | coded position           |
# +---------+--------+------+--------------------------+
#
# xmin,xmax,ymin,ymax,x,and y are stored as positions 
# relative to the coverage of the slippy map system
#
# x positions from 0 = 180 degrees west to 1 = 180 degrees west
# y positions from 0 = 85.0511 north to 1 = 85.0511 south
#
# note that these are in the mercator projection, i.e.
# dLat/dy is not constant
#
# encoding of x and y:
# the positions described above are divided by 2^31 for
# storage as an integer, 
# i.e.:
#  x: 0 = fully west, to 2^31-1 = fully east
#  y: 0 = fully north, to 2^31-1 = fully south
#
# integers are big-endian
# double-precision floats are platform-dependant

Testing

• sample tracklog view
• sample tile
• benchmark the tile
• test the GPX reader