From: merrett@ac.dal.ca
Newsgroups: comp.infosystems.gis
Subject: Re: Map Layers and Overlay
Date: 22 Jul 92 15:41:06 GMT
Organization: Dalhousie University, Halifax, Nova Scotia, Canada

In article <1992Jul21.135609.3112@cs.hw.ac.uk>, alia@cs.hw.ac.uk (Alia
Abdelmoty) writes:
> Hi,
> 
> I am currently involved in a project which studies modelling geographic data.
> 
> I have a question concerning the definition of a map layer and the use of
> the overlay operation from both the map user view (assuming that there are
> no modelling constraints) and the way these are handeled in current GISs.
.
.
.
        I don't know what the conventions are for this, but I've been working
on a basic GIS for Windows and have been thinking about groupings of data and
how to best implement it for speed/size/usefullness.  What I've been toying
with is a three level structure, composed of:

objects - The lowest level, essentially any one relatable structure, it could
          be a scatter of oil-wells, a polygon representing a house, any ONE
          relatable object.  So using the city metaphor, a house or tree or
          fire hydrant (plug) would be an object.  The definition would have
          to be able to handle three types of situation, as far as I can see:
          point data (i.e. oil well positions), line data (a rail-line or
          highway), and polygon data (any closed object like a house outline).

layer   - A series of relatable objects.  So the houses on a block could be
          made into a layer, or all the houses in a neighboorhood.  This
          structure should also be able to incorporate OTHER LAYERS.  So
          every block of houses could be defined as a layer, then the layers
          defining a ward (an arbitrary political division in Canadian cities
          which defines the area an alderman represents) could be grouped into
          a layer.

map     - This is a special case of the layer group.  It's essentially a
layer     "background" if you like.  It could be a bitmap, or vector image or
          be composed of one or more layers combined and would give the user
          an idea of their spatial position.  So in the city metaphor, this
          could represent the street grid, or the coastline on a coastal city,
          or whatever.  The only distinction is that once defined it is
          'invisible' to editing.  You could edit an individual layer or bitmap
          or whatever and re-link it to a map layer, but in the program this
          should be a static object which is kept copied in a separate area for
          fast refresh.

	Of course I'm speaking with complete ignorance of whatever standards or
conventions may be in common use, but I think this structure would work well
for vector data.  Bitmap data is another can of worms, however...  It can be
'underlain' in this setup, and potentially it could be accessible to the program
but not in a very sophisticated way.  Of course, the whole purpose of this 
program is to keep it SIMPLE.


								Doug
--------------
Doug Merrett
Department of Oceanography
Dalhousie University
Halifax, Nova Scotia
Canada

Internet: merrett@ac.dal.ca
              -or-
          merrett@mudchem.ocean.dal.ca
Comments: Gated by NETNEWS@AUVM.AMERICAN.EDU