Drawing boundaries is not that difficult, unless the area is a river estuary like 'The Biesbosch' in the Netherlands. How would you tackle this problem to get a decent boundary so all depth contours remain within the waterbody.

All ideas are welcome;

There's a few ways to go about this but I have access to some processing tools that no-one else does.

Internally, OSM doesn't represent oceans as polygons...instead, they represent them by a very larger collection of lines where each line is connected and has the water to the right of the line and the land to the left of the line.

Alot of other mapping entities represent water as collections of lines instead of areas as they are much easier to digitize but difficult to recombine. For that reason, recombination tools exist in most major GIS software packages.

You could digitize each ring and hole individually and then manually combine all rings into a single polygon. You could get some pretty crazy rings this way so splitting into sections might be necessary.

If the image didn't have text, I might even do this pixel by pixel using color band identification and later merge the millions of polygons. I actually wrote an internal tool for setting polygons around depth ranges in Digital Elevation Models (used to identify Sub-Zero Depth Areas in stuff like SRTM). Setting this to work on color bands would be a simple modification.

Hello Danny,


the tip of Greg is a good and a professional solution.
But i distrusting the OSM or topo chart quality. The comparison with the reality most show undesirable deviations (e. g. aerial photos, most problem: water area under trees, shallow water with denser vegetation, new constructions, own experience - up to 50 m). In Your case i would prefer draw with hand from aerial photos (e. g. GE). The time for this region is about one winter evening and 1/2 bottle of good wine ....


Best wishes an Merry Christmas for all
fjordfisher

Danny,

I put a tutorial on the old DrDepth forum a long while ago on how to deal with complex shorelines.

Simply create zero depth points along a shoreline vector spaced at 3-4m intervals and use only these. This dispenses with the "one boundary" problem as well. I do this in GM.

It works equally well in RM. Its the method I use for all my chart calculations.

My own preference for your deltas would be to simply layer the polygons rather than cut holes. Much simpler. Draw your initial shorelines by hand, or if you are really lazy do a screen grab, a bitmap to vector conversion, then georeference the final image.

T

Hi Danny,
you could do all little lakes and the big one in single steps and create your major and minor shapefiles step by step.

As a second step create your waters with only the shorelines step by step in RM and combine these outlines with QGIS to one single polys shapefile. With two lakes I had to manually draw a rectangle outline around my lakes. Save this new outline and let IMC generate your water with that outline and all your major and minor shapefiles.

edit: Just found this again ...
http://www.reefmaster.com.au/index.php/forum/feature-requests/37-export-boundaries-only#117

I tried this weeks ago for one map of two lakes, which contains additional islands. It also should work with lakes on an island, but I did not try yet.

Thank you boys for all your suggestions and tips. I will start this project very soon.

AnglingCharts wrote:
Hi Trevor, I couldn't find it on the Drdepth Forum. Could you point me in the right direction? Thank you.

Hi Danny,

I just had a look, and many of the files I put there have now gone. I couldn't find the tutorial at all. I think I have a copy here, so I'll put it on my website. It uses GM.

T

AnglingCharts wrote:
o.k. Trevor, thank you very much.