Wow, those really are "The Questions" in cadastral GIS! For what it's
worth, see below for my "short" answers:
On Wed, 8 Nov 2000 16:50:14 -0600, "David Hovendick"
>We are about to embark on a project of creating a Property/Subdivision
>Parcel GIS map that will be used by local city and county agencies for
>taxation and other purposes. We will be integrating our private survey data
>and drawings with registered parcels and subdivisions to create the map.
>Some of the issues of concern are:
>1) What do we do when descriptions don't fit existing measured boundaries?
They never will - expect it. Assuming that you're drawing the data by
reading and interpreting the recorded data you can use proportional
drafting techniques that compromise the distances and distribute the
relative error, forcing everything to fit between the section corners
or aerial occupation. Or you can use straight COGO/precision input
and place each parcel line with exact relative accuracy to itself (but
not those around it). If you use the first solution then the
surveying and engineering community will distrust and discount the
system. If you use the second method then you have to be prepared for
gaps and overlaps. Both require skill and experience on the mapping
technician, and a strong suite of software tools to support their
>2) Is there a need to use a predefined coordinate system and map projection
>in case other agencies desire to use the map? Or can we use our own local
>coordinate system and assume the map can be translated as the need arises
>without compromising the map integrity?
You'll be better served in the long run by using an established
regional coordinate system; state plane or UTM. It'll be easier to
incorporate other GIS layers as your system grows.
>3) The county tax office is using ESRI Arcview software and has a parcel
>database which we will need to link to the map. We will be needing to
>maintain this map as new surveys are conducted and subdivisions are created.
>Our survey drawings are done in Autocad Land Development Desktop, so we will
>be translating the data to Arcview and perhaps vice versa. What kinds of
>issues will we face when translating and updating the map?
That's pretty application specific, and I've no experience with that
AutoCAD application so I can't say. If you use AV's CAD reader then
it will bring in the graphics, and the database attributes, but it's
still a tricky operation to set it up. You should have good skills
with your DB app, ODBC, etc.. If you set it up right then the ArcView
side will take care of itself, but . . .
>4) The local city office will be utilizing the map for it's own purposes
>(i.e....utilities and properties). How can we keep all parties up to date as
>we will be compiling updates from ourselves and the other agencies, who will
>likely be doing their own editing of their maps.
You should be consistent with the application and the jurisdiction
that maintains the parcel maps. Other departments can draw their own
layers on top of the parcel map layer, but shouldn't be allowed to
change the parcel maps - just like you shouldn't be allowed to change
their's. If two departments start trying to make changes to the same
data layer, especially if it originates in a different format, then
you've opened the door to a nightmare.
>5) How can we protect our investment in the map so that all our time and
>resources are not freely distributed without our permission?
That depends on the Freedom of Information Act and your own state's
statutes regarding the distribution of public data. Some states
specifically exempt GIS data from FOIA, allowing the
county/municipality to charge for the data. Other states don't, and
the local governments can only charge for the cost of the media to
distribute it on. This is a huge topic right now, made all the more
complex by local government's acceptance of the internet.
>Lastly, we would appreciate any links to resources that will help us
>anticipate the problems we might encounter in such a project.
Yours are very good questions. You can fumble around and back into
these and the many other answers you'll need. Doing so you'll gain
your experience the hard way - by making lots of mistakes. I really
suggest you find a good local GIS consultant - cadastral mapping on
the east coast U.S. is a little different than the rest of the
country. They'll be able to help you with the right software to use,
vendors that produce customized tools designed for cadastral mapping,
and an implementation approach that will serve everyone's interests
while protecting your data's integrity and your investment. They're
not cheap, up front, but worth it when you consider how important
those first steps are. Ask them hard question - make them earn their
Hope that helps. Good Luck!