2 replies to this topic

[SenTnel]

Hello all! Im new to this map making business, and very interested to learn. My office planning department has basically two types of maps, shapefile format:

1- Layers with Street centerlines (polylines), and
2- Layers with City Blocks (polygons)

Im wondering, what is the workflow creating city maps? First we create centerline and from this centerlines create the city blocks or they are created in two different proccesses?

Do GIS software (we have ArcGIS 10 here at the office) generates city blocks (polygons) automatically, with the help of some sort of tool, from centerlines? Or after having created all centerlines do we have to start creating (drawing) all city blocks (poligons) within centerlines intersections?

Attatched you'll see two "samples", one is a small screenshot of centerline layer, and the other is a polygon layer of the same area.


Thanks for clearing my doubts !

[David Medeiros]

Technically speaking in the second image you are not creating city blocks but road right-of-ways (ROWs). These can be built using the buffer tool with a buffer distance determined by a road width attribute. Be sure to set the dissolve option to get a seamless ROW layer. You can use the intersect tool with say a city boundary file to get actual "block" polygons. But again these are not real city blocks, for that I think you'd need a parcel layer.

[Dennis McClendon]

It's a general principle of cartography to work from more detail to less detail; from larger scale to small scale. So usually the blocks are built to high accuracy, using parcel data and subdivision records. In addition, there may be a curbline layer created from orthophotos. Finally, centerlines are created within the street ROW as a way of generalizing this highly detailed dataset.

Taking generalized centerlines, ones not based on actual ROW or cadastral records, and then creating blocks is a funny way to work, and means you end up with theoretical blocks that are completely fictional, not based on anything based in reality.