Josh, Good point. You could just calculate the point density of the data set (points per square km) and find a relationship to map scale. Not perfect (for so many reasons), but might be adequate as a first approximation.
As for "at what scale does the topology become incorrect", well it's always incorrect. An improvement to the question would be "at what scale does the topology become intollerable to the user" (hey, this IS "geowanking"!). Somewhat subjective and dependent on the use of the map; a tough problem as you say. Edward Tufte gave some opinions in his book "Graphical Display of Quantitative Information" (dry title, exciting book). His three (or four?) books (http://www.edwardtufte.com/tufte/books_vdqi) should be mandatory reading for web mappers or any of the new cartogaphers. Brent Fraser GeoAnalytic Inc. Calgary, Alberta ----- Original Message ----- From: "Joshua Lieberman" <[EMAIL PROTECTED]> To: <[email protected]> Sent: Tuesday, May 08, 2007 9:25 AM Subject: Re: [Geowanking] Scale of vector data > Brent, > > You describe a useful approach. The pixel resolution approach can > also be valuable for judging maximum vector resolution. There isn't > much to rendering more than 1-2 nodes per pixel of resulting map / > screen / print image. Minimum resolution is much harder to judge, > since the irregularity of a feature may be so variable even within a > dataset. Could be just aesthetic, or otherwise application-specific. > For example, at what scale does the topology become incorrect (holes > not shown correctly, etc.)? At what scale does the vector > representation misrepresent feature length or area significantly? A > tough problem, I think. > > Cheers, > > Josh > > On May 8, 2007, at 10:43 AM, Brent Fraser wrote: > > > Ari, > > > > Usually the creator of the vector data would select a > > map scale (e.g. 1:50,000) that the vectors would be > > rendered, then ensure the positional accuracy and the > > resolution/point-density of the vectors would be suitable > > for that scale. Of course the end user of the data may > > choose to use it at a different scale, but it may appear too > > generalized (jaggy, lack points, etc) at a larger scale > > (e.g. 1:10,000), or have too many points (or features) at > > smaller scale (1:1,000,000). > > > > For raster image data (sorry, I don't have one for > > vectors), my rule-of-thumb for relating resolution to scale > > is: take the pixel size (resolution) in meters, multiply by > > 4000 and that will give a reasonable scale to view the > > image. So for Quickbird satellite image data with a > > resolution of 0.6 meters, a good scale would be 1:2,400 (0.6 > > x 4000=2400). Any larger scale than that and you'll start > > to see pixel edges. > > > > For vectors, perhaps a roughness value (using fractal > > math?) could be computed and used with the coordinate values > > to come up with appropriate scale. It would likely depend > > on the objects the vectors represented (streams are rougher > > than roads, etc). Just a thought... > > > > Brent Fraser > > GeoAnalytic Inc. > > Calgary, Alberta > > > > ----- Original Message ----- > > From: "Ari Jolma" <[EMAIL PROTECTED]> > > To: <[email protected]> > > Sent: Tuesday, May 08, 2007 3:35 AM > > Subject: [Geowanking] Scale of vector data > > > > > >> I'm a bit ignorant on this issue. I've thought that it's a > > bit > >> meaningless to talk about the scale of vector data but > > clearly one can > >> have more accurate and less accurate polygons for example. > >> > >> Are there commonly accepted methods for computing the > > scale of an > >> arbitrary geospatial dataset? > >> > >> Just asking, > >> > >> Ari > >> > >> -- > >> Prof. Ari Jolma > >> Geoinformaatio- ja paikannustekniikka / Geoinformation and > > positioning > >> Teknillinen Korkeakoulu / Helsinki University of > > Technology > >> tel: +358 9 451 3886 address: POBox 1200, 02015 TKK, > > Finland > >> Email: ari.jolma at tkk.fi URL: http://www.tkk.fi/~jolma > >> > >> > >> _______________________________________________ > >> Geowanking mailing list > >> [email protected] > >> http://lists.burri.to/mailman/listinfo/geowanking > > > > _______________________________________________ > > Geowanking mailing list > > [email protected] > > http://lists.burri.to/mailman/listinfo/geowanking > _______________________________________________ > Geowanking mailing list > [email protected] > http://lists.burri.to/mailman/listinfo/geowanking _______________________________________________ Geowanking mailing list [email protected] http://lists.burri.to/mailman/listinfo/geowanking
