Topology Overlay

The Topology Overlay dialog creates a results drawing from two other drawings using Identity, Intersect, Union and Update overlay functions. These functions may also be performed by a sequence of transform toolbar operators. The Topology Overlay tool provides one-step functionality to make it easy to perform these common functions. Overlays simultaneously create new objects in the results drawing and also populate data columns as necessary. Overlay functions are also referred to simply as overlays.

 

All overlays require two drawings: a data drawing that may contain areas, lines and points and an overlay drawing that must contain areas. The areas in the overlay drawing guide the operation of the chosen overlay function to create a new, result drawing.

 

The result drawing will inherit all columns from both the overlay drawing and the data drawing. There is no mapping between the columns in the overlay drawing and the data drawing. Each resulting object inherits all attribute values from the overlay drawing and from objects in the data drawing from which it has been produced. A topology overlay will automatically copy over values from active columns, rank columns, and columns brought into a drawing's table via relations.

 

·      Identity splits all areas, lines and points in the data drawing with areas in the overlay drawing and places each resulting part of the original object from the data drawing into the result drawing.

·      Intersect intersects all areas in the data drawing with areas in the overlay drawing and places each resulting part of the original area from the data drawing that lies in at least one area in the overlay drawing into the result drawing.

·      Union intersects all areas in the data drawing with areas in the overlay drawing and places each resulting part of the original area from the data drawing into the result drawing, then it intersects all areas in the overlay drawing with areas in the data drawing and places each part of the original overlay area that does not lie in any area in the data drawing into the result drawing.

·      Update intersects all areas, lines and points in the data drawing with areas in the overlay drawing and first places each resulting part of the original object from the data drawing that does not lie in any overlay area into the result drawing, then second placing each overlay area into the result drawing.

 

The above overlays are similar to those in various legacy GIS systems, except that they are not subject to some of the limitations that these GIS systems have. In particular, Manifold allows the data drawing to contain areas, lines and points (and not just areas as is the case with some systems) and overlays these objects simultaneously without forcing the user to perform the overlay operation once for each object type as would be required by some other systems. Manifold also automatically resolves overlaps between areas and other topological anomalies both in the data drawing and the overlay drawing, without forcing the user to go through a separate topology cleaning step.

 

Performing a Topology Overlay

 

1. Open the drawing to be used as the overlay drawing or data drawing, or open a map containing the drawing and set the drawing's layer active.

2. Launch Drawing - Topology Overlay.

3. In the Topology Overlay dialog, choose the Data drawing, the Overlay drawing and the Method. Press OK.

 

Topology Overlay Dialog Controls

 

When choosing object sets to use in the Overlay or the Data drawing, choices available will be all objects or any selection or saved selection in that drawing.

 

Data

Choose an object set in the data drawing upon which the overlay will operate.

Overlay

Choose an object set in the overlay drawing that will control the overlay. Only areas can be used to control the overlay.

Method

Choose an overlay method.

Force copy / copy transfer rules

When checked, ignores the transfer rules defined for each column and instead always applies a copy / copy transfer rule. Similar to how legacy GIS applications operate. Not checked by default.

 

Topology Overlay Methods

 

The following methods will appear in the Method box for topology overlays:

 

Identity

Split all areas, lines and points in the data drawing with areas in the overlay drawing and places each resulting part of the original object from the data drawing into the result drawing.

Intersect

Intersect all areas in the data drawing with areas in the overlay drawing and place each resulting part of the original area from the data drawing that lies in at least one area in the overlay drawing into the result drawing.

Union

Intersect all areas in the data drawing with areas in the overlay drawing and place each resulting part of the original area from the data drawing into the result drawing. Next, intersect all areas in the overlay drawing with areas in the data drawing and place each part of the original overlay area that does not lie in any data area into the result drawing.

Update

Intersect all areas, lines and points in the data drawing with areas in the overlay drawing and first place each resulting part of the original object from the data drawing that does not lie in any overlay area into the result drawing, then second place each overlay area into the result drawing.

 

Examples

 

Let's consider two drawings, A and B and the result of overlays between them.

 

images\eg_topo_overlay_a.gif

 

Drawing A consists of two areas.

 

images\eg_topo_overlay_b.gif

 

Drawing B contains three areas.

 

images\eg_topo_overlay_both.gif

 

If we show A and B together in a map we can see that the areas in A intersect the areas in B.

 

The following illustrations show the results of an "A operation B" where B is the Overlay drawing and A is the Data drawing. The results of each operation are shown as blue areas in a third results drawing shown in a layer above both the A and B drawings.

 

For example, we can use the Identity method with all objects in A as the Data drawing and all objects in B as the Overlay drawing.

 

images\eg_topo_overlay_identity.gif

 

Identity creates a result set of all the areas in A, but split up by their intersections with the areas in B. Instead of two areas in A, the result is eight areas in A. Each original A area has been split by its intersection with the three B areas to create three smaller areas plus a fourth area representing that part of the original A area not intersecting any B areas.

 

images\eg_topo_overlay_intersect.gif

 

Intersect creates six areas in the result set, three areas from both of the original A areas where they intersect the B areas.

 

images\eg_topo_overlay_union.gif

 

Union chops up all areas by their intersections between areas in A and B. It is, in effect, a dual "cookie cutter" operation performed using B areas to cut A areas and then using the results to cut B areas as well.

 

images\eg_topo_overlay_update.gif

 

Update preserves all B areas and moves them into the result. It takes that part of any A area not lying within an intersection with a B area and moves it into the result as well. We can think of Update as being an "updating" of any changes in A areas using B areas (the Overlay) as being an overriding control. For example, suppose we have some town boundaries that have been changed but we want areas that represent unchanged water bodies (such as lakes or ponds) to be preserved in any intersection of water bodies and new town boundaries. In that case we could make our changes in town boundaries and then do an Update using the water areas as the Overlay and know that whatever changes were made in town boundaries the water areas would "cookie cut" through any boundary areas as necessary so that water areas would remain invariant.

 

See Also

 

Traditional Topology Tools - An optional add-in that may be downloaded from the manifold.net web site. This add-in provides operations using stored topology.