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</o:shapelayout></xml><![endif]--></head><body lang=DE link="#0563C1" vlink="#954F72"><div class=WordSection1><p class=MsoNormal><span style='font-family:"Verdana",sans-serif'>Hello all,<o:p></o:p></span></p><p class=MsoNormal><span style='font-family:"Verdana",sans-serif'><o:p> </o:p></span></p><p class=MsoNormal><span style='font-family:"Verdana",sans-serif'>I'm quite new to GRASS (coming from ESRI and MicroStation) and have two questions concerning the vector data model. I've read GRASS Wikis, manuals and other docs from the web, but I could not find comprehensive answers yet. Sorry for the long text ... :-)<o:p></o:p></span></p><p class=MsoNormal><span style='font-family:"Verdana",sans-serif'><o:p> </o:p></span></p><p class=MsoNormal><span style='font-family:"Verdana",sans-serif'>First question: how are the different type determinations for geometry objects saved in GRASS? What makes a boundary a boundary and separates it from a line? The same for points and centroids.<o:p></o:p></span></p><p class=MsoNormal><span style='font-family:"Verdana",sans-serif'><o:p> </o:p></span></p><p class=MsoNormal><span style='font-family:"Verdana",sans-serif'>I've read about the theoretical differences (eg., boundaries are split at intersections and they make up closed rings). But those criteria can apply to lines as well.<o:p></o:p></span></p><p class=MsoNormal><span style='font-family:"Verdana",sans-serif'><o:p> </o:p></span></p><p class=MsoNormal><span style='font-family:"Verdana",sans-serif'>Command v.type is able to convert those types of geometry objects. That is, lines can be converted to boundaries and vice versa. But how does the system determine if an object is a line or a boundary? Where is that property saved? Is there an invisible attribute column or flag that contains the information? And how and when is that determination changed (if not by v.type)?<o:p></o:p></span></p><p class=MsoNormal><span style='font-family:"Verdana",sans-serif'><o:p> </o:p></span></p><p class=MsoNormal><span style='font-family:"Verdana",sans-serif'>Eg. when importing linestrings from DXF to GRASS with import option set to "centroid, boundary", the importer will produce lines as well (it will do this where linestrings have dangling errors). That means that there must be an automated way (running in the background) of determining the geometry type. What are the rules it follows?? As you can see, it does not do what the user chooses in the import options that were set. If that was the case, no lines would come out, just boundaries.<o:p></o:p></span></p><p class=MsoNormal><span style='font-family:"Verdana",sans-serif'><o:p> </o:p></span></p><p class=MsoNormal><span style='font-family:"Verdana",sans-serif'>Second: Up to now, I was not able to understand the concept of layers in GRASS. I've read a lot of explanations, most of them telling that a layer is a set of category values making up a different view to the underlying geometry objects. Using layers, the geometries can be aggregated or individually marked in a different, yet parallel manner at the same time.<o:p></o:p></span></p><p class=MsoNormal><span style='font-family:"Verdana",sans-serif'><o:p> </o:p></span></p><p class=MsoNormal><span style='font-family:"Verdana",sans-serif'>But everything described in the manual or Wiki seems to me like one can do this using different attribute columns as well. IMHO, that applies even to the Wiki example that was just produced to show the advantage of the layer concept over attribute columns (example with field borders that are paths at the same time).<o:p></o:p></span></p><p class=MsoNormal><span style='font-family:"Verdana",sans-serif'><o:p> </o:p></span></p><p class=MsoNormal><span style='font-family:"Verdana",sans-serif'>So question Nr. 2 is: what is the real advantage of the layer concept over a sophisticated set of attribute columns? And where is that layer structure saved physically? Can I make it visible? In the end, is it just a set of (most of the time invisible) columns??<o:p></o:p></span></p><p class=MsoNormal><span style='font-family:"Verdana",sans-serif'><o:p> </o:p></span></p><p class=MsoNormal><span style='font-family:"Verdana",sans-serif'>Thanks and regards, Uwe<o:p></o:p></span></p></div></body></html>