[Qgis-user] Preferred Scale of a Polygon Shapefile?

[Nicolas Cadieux]

Hi,

I think you already know it but  a well made GIS vector file should be drawn on a scale of 1:1. That means that a 1 km line like a road should mesure 1km in the GIS and not 100m.  Scaling in determined when printing the map.

But I understand your point as the amount of nodes in each line will determine the amount of details in the map as you say below.  It’s easier to automatically simplify a shape than to density a shape as that will only add new nodes but no new details. 

I don’t know of any tools that can figure out what you need.  You need a good eye and some knowledge of what scales do (educated guess).  As an example, on a printing scale ok 1:50 000 (or 50k) a 50 meter feature, like the a bay on a lake would measure 1 mm on the map so if you have a 50 m bay in reality but can’t see any nodes in the polygons or polylines, then the file is to course to be printed at that scale.  (You can extract the nodes from the files to help.) A tool could try to estimate the minimum size of a feature I guess. Perhaps there is a way to calculate the node m values to figure out the distance between a node and it’s predecessor node??? Anyone? You could also generate the area for each polygon.  That would give you an idea of the smallest polygons if you sort by size.

An other way is to look at the biggest river represented by a single line.  If a river that measure 50 meters wide is represented by a single line, then, that gives you an idea of the scale.  In theory, if you know he provenance of the map, then that info should be in the metadata or the file creation standards.  The problem is that you don’t always have that info.

Hope this helps a bit.
Nicolas 



> Le 6 sept. 2019 à 05:41, Tim Makins <tsg.makins at gmail.com> a écrit :
> 
> If a polygon shapefile has no included scale information, is there a QGIS tool that will assess it and determine the preferred range of scales that would be most suitable, or is this only done by eye? I have seen comments suggesting a visual comparison with features in files of known-scale, but hoped there might be some automatic tool.
> 
> Such a tool would assess the suitable scale for any polygon or line shapefile, as guidance to learn whether it can be incorporated into a map with other shapefiles who's scale is already known. I think this would be a very useful tool, and am wondering if it is already available.
> 
> Say, for instance, that I are making a map with shapefiles containing Medium scale data, 1:50m, from, say, Natural Earth (https://www.naturalearthdata.com/downloads/). I might wish to include another polygon shapefile, but if it is too coarse, it would look stupid, and if it is too fine, it would look out of place, and would be better simplified. The tool I am thinking of would be able to assess the unknown shapefile, and give a range of scales that it would be appropriate to use the unknown shapefile in: for instance from 1:40m to 1:60m.
> 
> 
> ---
> This email has been checked for viruses by Avast antivirus software.
> https://www.avast.com/antivirus
> 
> _______________________________________________
> Qgis-user mailing list
> Qgis-user at lists.osgeo.org
> List info: https://lists.osgeo.org/mailman/listinfo/qgis-user
> Unsubscribe: https://lists.osgeo.org/mailman/listinfo/qgis-user