Contour provision is a major enhancement for this version of oomap – there are globally-available options for contours along with locale-specific higher-quality options.
Each contour option will select a different contour rendering for the final map. Some of these options produce a visible preview in the web view, while others do not (due to differences in how they are created).
Apart from “OS (10m)”-derived contours, rendered contours all show the direction of slope using downward tags at regular intervals.
As well as “None” there are two major classes of contour:
- Global, satellite-derived contours. No pre-rendering is available.
SRTM (5m), SRTM (10m), GLO-30 (5m), GLO-30 (10m).
- Local, pre-calculated contours. Pre-rendering is available.
OS (10m), LIDAR (5m), LIDAR (10m).
“SRTM” (Shuttle Radar Topology Mission) contours are available at 5m and 10m vertical intervals, and are derived from a near-global digital elevation model collected by the Space Shuttle in 2000. Data does not extend beyond latitudes 60 degrees N and 60 degrees S.
“GLO-30” contours are also available at 5m and 10m vertical intervals and are derived from an almost complete global digital elevation model (Copernicus GLO-30) from data collected between 2011 and 2015.
Both datasets have a nominal horizontal resolution of 30m but the GLO-30 set seems to be of higher overall quality (and is more recent). However, both suffer from land-cover effects – since they are measuring the radar-derived average heights of the ground, they are affected by tree cover and buildings, in both cases giving artefactually higher readings than expected for the “true” terrain.
Because these datasets are global, they are large and also take considerable resources to transform from height data into contours. Therefore for both, contours are generate on-the-fly when a map rendering is requested. First, a 1 degree square portion of height data is downloaded if not already available locally, then contours are calculated just for the requested map area. This process can add 30-60 s to a typically map rendering job – so be patient!
OS (10m) – These are contours from the Ordnance Survey, covering Great Britain with high-quality but relatively low resolution (50m horizontal grid) data.
LIDAR – Where other high-resolution digital terrain models (typically produced by airborne LIDAR surveys) are easily available and appropriately licenced, these have been downloaded and pre-processed to generate contour datasets that can be rendered at 5m or 10m vertical spacing. Current coverage includes:
- Most of England and Wales (~ 98% coverage). This is a high-quality dataset that has been post-processed to remove many artefacts, with high horizontal resolution that has been downsampled to 10m horizontal precision.
- Some of Scotland, covering most of the population centres and more
- Northern Ireland
- New Zealand (major population centres only)
- Australia (major population centres only)
- Canada (parts of Ontario)
Other datasets could potentially be similarly processed and made available – please leave feedback if you know of such a source and would find the data useful.
Copyrights for the contour sources are printed on the final map rendering.