This is a rather great response to the burning issue of topology that
visits our list from time to time - "How do I use MapCheck or get a hold
of it?"
I note the need of AkeView users to get their own MapCheck know in ESRI
circles as XTOOLS and/or AkeInfo.
FYI
MidNight Mapper
aka Neil
************** from Akeview-l*****************
ORIGINAL QUESTION:
Basically I have a very simple question. Does ArcView GIS
have topology?
I was under the impression that every vector based GIS has
topology so that spatial queries can be performed. However,
recently I found out that one needs to perform CLEAN and
BUILD operations to create topology. Since ArcView does not
have CLEAN and BUILD, does that mean that ArcView does not
have topology?
Do shapefiles created in ArcView have topology? If yes,
then why don't they show topological attributes such as
FromNode, ToNode, etc.
When an Arc/Info coverage is added to ArcView, can ArcView
retain the underlying topology?
If this question has already been discussed here, please
excuse me because I am re-subscribing after a long time.
Thanks for your attention
Best regards
Uzair Shamsi
[EMAIL PROTECTED]
SUMMARY
I received 22 responses. An overwhelming majority (17 or
77%) indicated that ArcView does not have topology.
However, many respondents from this group agreed that
ArcView can create topology on-the-fly on an "as needed"
basis. The best "Yes" (AV has topology) answer came from
Kim Ollivier. The best "No" (AV does not have topology)
came from John Harner. My summary based on the list
responses and my own research is as follows:
1. Some systems (ARC/INFO) perform computationally intensive
topological relationship calculations up front and store
that information as attributes (fnode, tnode, lpoly, rpoly,
etc.)
2. Some new systems (ArcView shapefiles, MicroStation
GeoGraphics, etc.) compute topological relationships on the
fly when needed
� Data is never out dated
� Speeds display
3. It is not necessary for a GIS to have [stored]
topological attributes to qualify as a topological GIS.
� ArcView is a topological GIS
MANY PEOPLE REFERENCED A GIS WORLD ARTICLE (NUTS AND BOLTS -
SHAPEFILES, MAY 98). UNFORTUNATELY I DO NOT SUBSCRIBE TO
GIS WORLD. I WILL APPRECIATE IF YOU HAVE THIS ARTICLE AND
CAN FAX IT AT 412-269-5865, ATTENTION: UZAIR SHAMSI. THANKS
All the responses are listed below.
ARCVIEW HAS TOPOLOGY
1. Best "Yes" Response from Kim Ollivier: This is a very
important question that has been soft-peddled by ESRI to
avoid alienating their existing user base. Its a bit like
the ministers in the church having a much more sophisticated
view of their religion than their parishioners, but they go
along with the old religious concepts that the congregation
still have that could well be discarded. I was one of
them... (Q) Basically I have a very simple question. Does
ArcView GIS have topology? (A) Not stored topology.
Shapefiles are object-record based. ARC/INFO coverages are
stored as "decomposed structures" which we traditionally
regard as "topology". It is a planar enforced scheme. Shape
polygons are defined by a boundary of lines. Each shape has
a complete boundary defined. Nothing is stored to indicate
adjacent polygons, but there are requests to find out when
you need to know. Topology is generated on the fly as needed
for the features needed. There are many other topological
concepts that could be used to describe polygon features
that cannot be handled by ARC/INFO data structures directly.
These have been incorporated in SDE and ArcView. Try out the
methods available in the spatial selection tool. Concepts we
have never dreamed of in ARC/INFO are there. Such as a
"spatial join" on the shape "field". (Q) I was under the
impression that every vector based GIS has topology so that
spatial queries can be performed. However, recently I found
out that one needs to perform CLEAN and BUILD operations to
create topology. Since ArcView does not have CLEAN and
BUILD, does that mean that ArcView does not have topology?
(A) Well I used to think that too! But just consider that
you can open a native AutoCAD drawing file and perform
spatial queries. There is no claim to topology by AutoCAD
for drawings. So ArcView can create topological
relationships "on the fly". Therefore you can claim that
ArcView *does* have topology. Actually ArcView *does* have
CLEAN! And all the spatial operators that you expect in
ARC/INFO. They have been brilliantly put together in a set
of menus in the XTOOLS extension that everyone should have.
Of course being object oriented the commands work on a pair
of objects instead of a whole layer. That means you have to
create a loop to process each pair. Its second best for some
things that you are expecting, but first for others such as
history tracking, non-contiguous polygons, indexing, and
storage. (Q) Do shapefiles created in ArcView have topology?
If yes, then why don't they show topological attributes
such as FromNode, ToNode, etc. (A) Well each shape has
topology. Its just that polygons and lines are not
decomposed. Topology is build on-the-fly for network
analysis that you cannot see. There is still a shape.flip
command. Its just that there are other ways of defining
topology other than the planar view of polygons. Consider
regions where there are overlapping polygons. You cannot see
left and right then, but still do spatial operations because
the commands build topology on the fly. A similar thing
happens with shapes. I think its a bit tough for users to
program, and its is very very slow. A backward step for
spatial analysis but a great leap if you only want to store
shapes that are equivalent to records in a database with no
complicated relationships to other features. (Q) When an
Arc/Info coverage is added to ArcView, can ArcView retain
the underlying topology? (A) Yes, of course, because ArcView
cannot edit coverages. The view is different because each
theme has to have only one type, so lines are treated like
polylines and polygons like regions. What you may mean is
that if you *translate* a coverage to shapes what will
happen. Well the attributes are retained but the tables
linking the lines and features are not. Nodes are treated as
points, polylines do not have nodes. As far as editing
polygon shapes. Yes you have to edit two lines on every
common boundary, and ArcView has added functions to do that
for you, common vertices will be snapped to keep each
polygon filling the gap.
2. It has topology but the shape file model is different
from the ArcInfo cover model. In ArcView you had to clean
and build, which performed a lot of checks and corrections
when possible. ArcView does not require that but it still
has topology. It has tools that emulate clean, dangle,
noderror and other traditional GIS functions but you are not
forced to run them. Geocoding required from-to nodes and
line direction so AV has the ability use these features, but
does not force the user to work with them. With ArcView you
can easily bring in an AutoCAD drawing of polygons, add
attribute data and use it. It might have dangles and misses
but you can use it. Since I usually create only point data
and my polygon base maps come for a state agency I have
never had to try to figure out clean & build in ArcView. The
new Geoprocessing extension in AV 3.1 allows clips,
dissolves, unions, mapjoin etc without forcing clean and
build. It may not be perfect but it is orders of magnitude
faster. I have no idea how it handles dangles, overshoots
and sliver polygons. ESRI seems to be moving toward putting
all ArcView function into AV.
3. You can build topology with the build theme script
located in the sample script directory for ArcView.
4. Yes, ArcView does have topology, but the topology is
built dynamically. ARC/INFO coverages are basically topology
which requires a lot of overhead to store. Shapefiles
themselves have no topology, nor do most other data formats
other than ARC/INFO coverages.
5. AV calculates spatial relationships, adjacency,
connectiveness, etc. "on the fly". That is all
relationships are determined from calculating the coordinate
geometries of the features. This basically allows AV to
have a more streamlined data structure without all the
baggage of the AI structure. This speeds display and some
analysis functions/calculations, but sacrifices other
functionality which is based strictly on topology. AI
performs many of the computationally intensive relationship
calculations up front and stores that information (i.e.
area, lpoly, rpoly, tnode, etc.). AV must recalculate these
each time.
ARCVIEW DOES NOT HAVE TOPOLOGY
1. Best "No" Response from John Harner: ArcView is a
non-topological model, i.e., there is no topology stored. I
had a similar question about 2 months ago about how AV
calculates spatial relationships without topology.
Basically, the answer is it either uses simple coordinate
geometry to determine overlapping features, or it calculates
topology "on the fly" as needed. Below is the SUM of my
responses: (1) AV calculates spatial relationships,
adjacency, connectiveness, etc. "on the fly". That is all
relationships are determined from calculating the coordinate
geometries of the features. This basically allows AV to
have a more streamlined data structure without all the
baggage of the AI structure. This speeds display and some
analysis functions/calculations, but sacrifices other
functionality which is based strictly on topology. AI
performs many of the computationally intensive relationship
calculations up front and stores that information (i.e.
area, lpoly, rpoly, tnode, etc.). AV must recalculate these
each time. (2) Also this from Tricia Maggio: There's an
article on shapefiles in the "Nuts and Bolts" column (by
Eric Strand)in GIS World, May 1998. I think it answers your
questions...
2. Simply put, the shapefile format has no topology. Only an
ArcInfo coverage has topology. That's one of the reasons you
will find that polygons in a shapefile don't share a common
boundary, there are two separate arcs, one for each polygon.
3. ArcView is a non topological GIS software. Spatial
analysis functions are calculated "on the fly" relying on
coordinate information in order to derive the spatial
relationships between entities (i.e. topology).
4. The spatial relationships between connecting or adjacent
coverage features (e.g., arcs, nodes, polygons, and points).
For example, the topology of an arc includes its from- and
to-nodes and its left and right polygons. Topological
relationships are built from simple elements into complex
elements: points (simplest elements) and arcs (sets of
connected points) are used to represent more complex
features such as areas (sets of connected arcs). Shapefiles
do not explicitly record topology.
5. No ArcView does not have the capabilities to create
topology, if you notice in your shapefiles lines and node
are in separate shapefiles. If you delete a line from a
shapefile the nodes are still there. I am not sure ArcView
retains underlying topology from ArcInfo, because it breaks
up node to arc information.
6. The Shape file format is not inherently topologically
consistent. It is possible to build a Shape file such as
polygons which have overlap or gaps, i.e. sliver polygons.
Depending on how you add features to a shape file, they may
not share exactly the same border, in the case of polygons,
or end points may not match, in the case of lines. This is
where the clean operation is critical. It can correct for
overshoots, undershoots, slivers, etc. The build operation
builds the topological references included in the AI
coverage format. By having this information internal to the
file format, certain operations such as calculating
adjacency or connectiveness can be quickly performed. AV
can still perform these operations, but it has to calculate
the relationships between entities on the fly. It is
basically a trade off. Shape files are smaller and less
cumbersome to display and work with, coverages can be more
efficient for analysis. Remember, all files will have
topology....there are relationships between entities. What
cleaning a coverage does is creates a topologically
consistent data structure. AV files may not be
topologically consistent, i.e. have slivers, overshoots,
undershoots, pseudo nodes, etc.
7. As far as I know, ArcView shape files do not have
topology. You can get topology on shape files by bringing
them into Arc and doing the build's and the cleans. This is
one thing that ESRI fails to mention to everyone. So their
spatial analyses are really just comparisons between graphic
files for the most part. If you find another answer
confirming topology, let me know. I have spoken to ESRI
reps asking the same question and they do confirm that
ArcView does not build topology.
8. The topology in ArcView is not implicit to the data
model. It is in ArcInfo. This means that when ArcView
needs topology for spatial operations it creates it as
needed.
9. ArcView does not have topology. It does have vector
based arcs, but no topology. It is sort of like a
blind-folded man standing in a room. If you hold an arm
out, you know it begins from the body, and extends in a
direction. But, you do not know what is connected to it at
either end. You do not know whom else is in the room. That
is why ArcView does not do anything with the from and to
nodes information. It may be carried down from ARC/INFO, or
any of the "higher" software, but not of itself. The
shapefiles only provide the basis of vectors, and ends
there.
10. ArcView does not support topology. If the topological
relationships are present in the data (as with an Arc/Info
coverage), though, they are accessible logically: for
example, the lpolyl# and rpoly# items, present in the arc
attribute table (AAT) of a coverage, and which refer to
records in a polygons attribute table (PAT), are conserved
when the coverage is converted to a shapefile and are
logically accessible. But those relationships are not
managed if the shapefile is edited in ArcView (unless one to
manage the topology vis-a-vis a lexical edit, which could be
tedious). Perhaps this is the major distinction between the
two products: ArcView as a "viewer" (that includes every
kind of analysis), and Arc/Info the tool that would be used
for developing and maintaining the source data.
11. a very simple answer: If you want topology you need
ArcCAD ( as I've seen) where you can
find...............topological attributes such as FromNode,
ToNode, etc.
12. ArcView does not generate topology, which is one of its
major limitations. You can only build topology in Arc/Info.
Yes, it does retain the topology of A/I coverages you bring
into ArcView, however, if you want topology on shapefiles
created in ArcView, you must bring them into A/I using the
Shapearc command. Once converted to an A/I coverage, you can
run the build or clean commands on the coverage to create
topology. Then, if you want, you can convert the coverage
back to a shapefile using the Arcshape command. Or you can
just use the A/I coverage in ArcView. The major advantage to
converting the coverage back to shapefile is portability.
You don't have to export shapefiles as you do with coverages
when you want to move them around on your system.
13. Not quite. Check the list archives. This question was
summarized a couple months ago. Also, if you have it,
there's a column on shapefiles in the May 1998 issue of GIS
World (Nuts and Bolts, by Eric Strand.)
14. I don't ArcView does topology. Shape files are
non-topological. There is a vendor that does topology (e.g.,
clean and build as you wish) for ArcView: Geokinetic
Systems.
15. No - the shapefiles don't store it explicitly Yes - its
possible to develop it - for example, we have a dangling
node detector which runs very quickly on large data sets. We
also have polygon construction tools. In Avenue, we can do
all of the topological queries and checks required. We
expect to announce a full set of topological management
tools for AV in the New Year.
16. ArcView does not have topology. Shapefiles are
non-topological datasets.
17. There is an extension that includes tools to build
topology within ArcView. Attached are two files that
describe the extension, including a review from the Sept.
1998 issue of GIS World.
==================
Thanks to all the respondents.
Best regards,
Dr. Uzair M. Shamsi, Ph.D., P.E.
Senior Technical Manager
Chester Engineers
600 Clubhouse Drive
Pittsburgh, PA 15108
Phone: 412-269-5907
Fax: 412-269-5865
E-mail: [EMAIL PROTECTED]
Home Page: http://www.pitt.edu/~ushamsi/ushamsi.html
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