<html><head><meta http-equiv="content-type" content="text/html; charset=utf-8"></head><body dir="auto"><div>The reason? The map itself is an abstraction. What size of details you will draw? Way not smaller? Etc. etc. </div><div id="AppleMailSignature"><br></div><div id="AppleMailSignature">What about thermal and hypogenic caves? </div><div id="AppleMailSignature"><br></div><div id="AppleMailSignature">There is no sense for in this discussion. </div><div id="AppleMailSignature"><br></div><div id="AppleMailSignature">Martin S. <br><br>Odesláno z iPhonu</div><div><br>11. 6. 2018 v 9:03, Nikita Kozlov via Therion <<a href="mailto:therion@speleo.sk">therion@speleo.sk</a>>:<br><br></div><blockquote type="cite"><div><div dir="ltr">
<div dir="auto" style="font-size:12.8px;text-decoration-style:initial;text-decoration-color:initial">> (5) "cave length": in the (old) manuals i studied that the cave length is "computed" measuring the length of an ideal line that goes thru the middle of the passage.</div><div dir="auto" style="font-size:12.8px;text-decoration-style:initial;text-decoration-color:initial">> now it is rather cumbersome to compute that line</div><div dir="auto" style="font-size:12.8px;text-decoration-style:initial;text-decoration-color:initial"><br></div><div style="font-size:12.8px;text-decoration-style:initial;text-decoration-color:initial">In Ukraine we are using term "Projected Cave Length", which is length of middle line of cave plan, for more or less horizontal caves,</div><div style="font-size:12.8px;text-decoration-style:initial;text-decoration-color:initial">or middle line of extended elevation for vertical ones.</div><div style="font-size:12.8px;text-decoration-style:initial;text-decoration-color:initial"><br></div><div style="font-size:12.8px;text-decoration-style:initial;text-decoration-color:initial">In practice good middle line approximation is skeletal line of 2d shape, computed on its vector or raster representation.</div><div style="font-size:12.8px;text-decoration-style:initial;text-decoration-color:initial"><br></div><div style="font-size:12.8px;text-decoration-style:initial;text-decoration-color:initial">For doing such for a vector data there are some scripts for AcrGIS by A. Grachev: <a href="http://speleo.land.kiev.ua/surveying-cave.html#2-5">http://speleo.land.kiev.ua/surveying-cave.html#2-5</a> </div><div style="font-size:12.8px;text-decoration-style:initial;text-decoration-color:initial"><br></div><div style="font-size:12.8px;text-decoration-style:initial;text-decoration-color:initial">Also Global Mapper has feature to compute middle line. </div><div style="font-size:12.8px;text-decoration-style:initial;text-decoration-color:initial">So when dealing with therion output i.e. plan in kml format it is possible to compute projected cave length in Global Mapper.</div><div style="font-size:12.8px;text-decoration-style:initial;text-decoration-color:initial"><br></div><div style="font-size:12.8px;text-decoration-style:initial;text-decoration-color:initial">If your input to calculate projected cave length is raster monochrome mask of cave internal area in any projection (plan|ext)</div><div style="font-size:12.8px;text-decoration-style:initial;text-decoration-color:initial">than you can use thinning algorithm to produce single-pixel line and then vectorize it.</div><div style="font-size:12.8px;text-decoration-style:initial;text-decoration-color:initial"><br></div><div style="font-size:12.8px;text-decoration-style:initial;text-decoration-color:initial">There are 2 apps from GRASS Gis to do such - namely r.thin (<a href="https://grass.osgeo.org/grass70/manuals/r.thin.html">https://grass.osgeo.org/grass70/manuals/r.thin.html</a>) and </div><div style="font-size:12.8px;text-decoration-style:initial;text-decoration-color:initial">r.to.vect (<a href="https://grass.osgeo.org/grass70/manuals/r.to.vect.html">https://grass.osgeo.org/grass70/manuals/r.to.vect.html</a>)</div><div style="font-size:12.8px;text-decoration-style:initial;text-decoration-color:initial"><br></div><div style="font-size:12.8px;text-decoration-style:initial;text-decoration-color:initial">In the 2010 I and A. Vechenko implemented own app to automate cave projected length calculation using raster data:</div><div style="text-decoration-style:initial;text-decoration-color:initial"><span style="font-size:12.8px"><a href="https://bitbucket.org/ngry/cavevaluer/src/CaveValuer/">https://bitbucket.org/ngry/cavevaluer/src/CaveValuer/</a></span><br></div><div style="text-decoration-style:initial;text-decoration-color:initial"><span style="font-size:12.8px">Raster gets thinned and then vectorized in the way similar to use r.thin + r.to.vect, but in gui app and with different thinning algorithm.</span></div><div style="text-decoration-style:initial;text-decoration-color:initial"><span style="font-size:12.8px"><br></span></div><div style="text-decoration-style:initial;text-decoration-color:initial"><span style="font-size:12.8px">Certainly raster-based approach has some drawbacks, such it is unable to do proper calculation for overlapping passages </span></div><div style="text-decoration-style:initial;text-decoration-color:initial"><span style="font-size:12.8px">(GlobalMapper capable to handle overlapped plans to compute centerline),</span><br></div><div style="text-decoration-style:initial;text-decoration-color:initial"><span style="font-size:12.8px">you need to do a post-process near entrance areas and so on, but it has significant advantage - raster skeleton can be computed </span></div><div style="text-decoration-style:initial;text-decoration-color:initial"><span style="font-size:12.8px">very fast, compared to vector-based algorithms (cavevaluer is not the fastest one, it is possible to do factor 100x faster on GPU).</span></div><div style="text-decoration-style:initial;text-decoration-color:initial"><span style="font-size:12.8px"><br></span></div><div style="text-decoration-style:initial;text-decoration-color:initial">Thinking about how it should be done in general, not relating to projections - is to compute 3d skeleton from 3d cave model,</div><div style="text-decoration-style:initial;text-decoration-color:initial">then we will have a good length approximation in all meanings.</div><div style="text-decoration-style:initial;text-decoration-color:initial"><br></div><div style="text-decoration-style:initial;text-decoration-color:initial">For convenience, attaching our with A. Verchenko thesis about the raster algorithm in Ukrainian language.</div></div>
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