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| GeoCommunity Mailing List |
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| Mailing List Archives |
| Subject: | [gislist] SUMMARY: Linear Referencing |
| Date: |
12/21/2005 05:50:01 PM |
| From: |
Hargis, Amanda |
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Thank you, everyone who emailed and phoned with ideas and thoughts about
getting started in linear referencing. The GIS community is truly
filled with some very generous people.
Here is a summary, with my original questions first, the answers to my
questions listed next, concluded by some additional suggestions. The
most frequent answer to my questions was, "It depends!" We'll
definitely be following up on your suggestions about project planning,
further research and people to contact.
Original Questions:
1. How do we choose route lengths? Should these be long or short? As
an example potential route, we have a highway that traverses the county
from north to south, passing through two incorporated cities as well as
unincorporated county. Should each stretch be a route, or should the
entire length be one big long route?
2. How do we choose a route ID? Should this be a randomly generated
number, or a value that matches a key field in a transportation
maintenance table we have?
Answers about Length of Route:
1. It depends on the applications you will use it for.
2. It depends on how the existing roads are numbered - by mile
marker? If so, where do the miles start?
3. Shorter is better, so that interpolation of distance will be
more accurate.
4. Longer is better, with the data determining the length - e.g.,
perhaps split the segment when the speed limit changes
5. Segments need to be broken at junctions with other segments for
routing (snow plow routes, etc.)
6. Set up length based on "character" or "utility" rather than
"long" or "short" - it should be an identifiable thing
7. Break the segments at intersections
8. It depends on who will use the data, and how
9. Longer is generally better
10. It depends on how the routes will be used - find a model that
allows you to use multiple road names (e.g., Hwy. 287 is also Main
Street)
11. Base the length on the usual method of identification for that
road and break the segments where the road name / address scheme
changes.
Answers about Choice of RouteID:
1. It depends on the correspondence of the tables to the features.
What if your database ID changes? Then the correspondence to the route
will be lost.
2. RouteID has to match a key field in an event table
3. RouteID should be automatically generated by the GIS
4. A DBMS guideline is never to associate a record ID with any
field in the database
5. It depends on what defines the routes as unique things, and how
you use them
6. A randomly generated number, so performance is fastest through
indexing
7. Random to enforce uniqueness
8. Perhaps match it to County Road ID
9. It depends on how the system is used. Design them to
accommodate the road data you will encounter.
10. User-ids link the routes to external tables, and internal ids
link the routes to sections, etc.
11. Make it a long enough number that there won't be conflicts (10 -
12 digits) and base it on the addressing grid / milepost system
Suggestions on who to chat with:
1. City of Boulder
2. Adams County
3. Various consultants
4. Neighboring counties - coordinate!
5. Colorado DOT - coordinate! Use their routes / IDs?
Suggestions on reading material:
http://support.esri.com/index.cfm?fa=downloads.dataModels.filteredGatewa
y&dmid=14
http://www.dot.state.ia.us/gis/lrs_project.htm
http://www.esri.com/news/arcuser/index.html : July-Sept. 2002
and Oct. - Dec. 2002 issues
Suggestions on Event Tables:
1. Make one big event table, with each record holding an Event Type
and an Event Value (e.g., event type = Name: event value = Arapahoe)
2. Make lots of event tables, each with a single focus (e.g., Road
Name events, Pavement Type events, etc.)
3. Make two event tables: one for static data and one for dynamic
data, since in databases, you model each type of data differently
Thanks,
-Amanda
.........................................
Amanda Hargis
GIS Coordinator, Boulder County, Colorado
303-441-3958 (voice) 303-441-3983 (fax)
ahargis@co.boulder.co.us
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