[OSM-talk] Coastlines

Andy Robinson Andy_J_Robinson at blueyonder.co.uk
Tue Mar 21 16:55:51 GMT 2006

Simon Hewison wrote:
>Sent: 21 March 2006 13:03
>To: talk at openstreetmap.org
>Subject: Re: [OSM-talk] Coastlines
>David Groom wrote:
>> Thinking forward to the Isle of Wight workshop, but also relevant
>> elsewhere, is the issue of coastlines.
>> Two issues spring to mind.
>> Firstly is it necessary to have coastline data in OSM?
>> Secondly if the answer to the first question is yes, then how should the
>> coastline data get into OSM?  It will not always be possible to walk /
>> drive along a coastline either because the land may be private, or
>> because it may be inaccessible due to woodland, cliffs etc.  It
>> obviously would be possible to use the Landsat imagery and trace the
>> coastline for an area that was being mapped, but if the
>> orthorectification of the Landsat imagery is not accurate then this is
>> not ideal.  Also it breaks the idea of having all OSM data derived from
>> user entered GPS data.
>Think how Ordnance Survey do this sort of thing. They don't go climbing
>all the cliffs with theodolites and GPS equipment. The traditional way
>of doing this is with trig. points and noting positions of headlands,
>inlets and the like.
>These days they use aerial photography, and with ortho-rectify them
>against known features that they have really accurately surveyed on
>land, and then trace the outlines. To get the mean high water and mean
>low water marks, you really need to take footage at those times of a
>suitable day.
>If someone wants to lend me a light aircraft with proper aerial
>photography gear, and fuel it, I'd be happy to fly it straight and level
>over the Isle of Wight a few times. The sort of aerial photography you
>get just by sticking a normal camera against the window is going to be
>at the wrong angle and a pain to align properly with other strips.
>The professional aerial photographers have one or more downward facing
>medium-to-large format cameras with the lens sticking out of the bottom
>of a specially modified aircraft, the modification itself needs to be
>certified as airworthy. Certainly, if anything protrudes from, or
>affects, the body of a certified aircraft that affects the aerodynamics,
>it will need to undergo certification.
>The US National Geodetic Survey contracts the work out and the details
>of how they do it are at
>Anyway, we *could* use landsat to do this, but we'll probably want to
>make sure that it's properly orthorecified against obvious features that
>OSM contributors have surveyed accurately on the ground.

In reality that's how you do aerial survey anyway. I'm used to working in
remoter cities of the USA and I regularly come across large white crosses
painted on the road for a project I am working on. These crosses are survey
control points and are manually referenced on the ground in relation to each
other. The area photography is then rectified to the resultant wire frame
and a final digital map produced from the aerial photo images. 

>Simon Hewison
>talk mailing list
>talk at openstreetmap.org

Andy Robinson

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