Tuesday, 29 March 2011

A l'eau - c'est l'heure

Some of the old Bartholomew maps show a limited amount of information on the depth of the sea around the coast. For my first attempts at emulating this I just created a low-water line from the Ordnance Survey data for Local government Boundaries (which mainly extend to the low-water mark).  It looked about right, but it seemed like a bit of a fiddle.

I thought I ought to be able to do better, so I set off to see what data I could find on the depth of the coastal waters around Britain. I found what I thought I wanted on the Emodnet site. And so began a long tortuous process of trying to understand how to get this data into a format that I could process and add to my rendering.

I'd used SRTM2OSM to calculate the land contours, because for those I was starting from SRTM data, and it seemed to be an easier solution than using GDAL. The Emodnet data isn't in the same format, so this all turned into a bit of an exercise in understanding different data formats, and how to convert and manipulate them using the GDAL toolset.

After a lot of effort I've ended up getting something working. The results aren't entirely convincing, but after a lot of false starts I've ended up a bit surprised that I could get anything at all produced.

The results for the Farne Islands, off the Northumberland coast, are shown above.

I've also rendered some larger versions covering more of the coast around  Northumberland, and the South Lakes.

Meanwhile, back on land, I have also generated a map of my local area, up to the  Chilterns. I've tried using an extract of this as a road map. It certainly has limitations, but it is usable, up to a point. Printing off extracts at 300dpi produces roughly the right scale for 1/2" to the mile. I don't think it would be wise to recommend any of these to a sailor though.


Chris Hill said...

The latest OS Opendata VectorMap District now has a set of polygons around the coast showing the foreshore between high and low water mark. http://blog.ordnancesurvey.co.uk/2011/03/os-vectormap-district-graduates-to-beta-release/

gom1 said...

Thanks Chris - I didn't know that. I'll have a try with that next....

Paul said...

What is the chart datum for the depths?
Judging from this
it looks like it is probably LAT (Lowest Astronomical Tide)?
If so, this is the same datum as UKHO charts.