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I am a very basic user and dont know the lingo to get to the gist of my question so some background might help.

Ive volunteered to create a pre-incident planning system for my rural fire department. Our primary vulnerability are extremely long un-mapped driveways.

So far Ive successfully transferred GPS data of the driveways over aerial photography of the district as bright green lines(a .SHP layer) with a label showing the length of each driveway. It looks great and is very useful as is.

Now the question: I want to draw the driveway vector lines as dashes or with markers set at 100-foot lengths. Our supply-line hoses come in 100-foot lengths; A vector line defined with markers at 100-foot intervals would save me a lot of time and improve the data immeasurably.

So far I have tried "Single Symbol" as a "Simple Line" using a customized dash pattern and the "Marker Line" using the interval placement. Neither of these options maintain the proper length while zooming or "scaling" (if that's the term).

Perhaps it is impossible? (at least at my level of knowledge. I taught myself this far and dont have a lot more time for fooling around :))

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Welcome to the site! Quick question: what software are you using for this mapping? ArcGIS (and which version)? QGIS? Mapinfo? Depending on what you're using there are a few different ways to do this. –  om_henners Jan 7 '13 at 3:46
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If you are working in QGIS I have just opened this for you hub.qgis.org/issues/6953 –  Nathan W Jan 7 '13 at 4:24
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3 Answers

As QGIS also has Simple and Marker Line I will answer from a QGIS point of view. Like Stephen said using the marker lines won't really give you want you need - although that would make a good feature request - which I have just opened.

You best bet at the moment to generate points along the line. I have outlined how you can do this in QGIS on my blog at Generating chainage (distance) nodes in QGIS

Luckly one of the guys on the project has taken my logic and made it into a plugin at https://github.com/mach0/qchainage.

To install the plugin

  • Click the ZIP button on the https://github.com/mach0/qchainage or follow this link
  • Extract the zip file into {yourusername}\.qgis\python\plugins\. You should now have a qchainage-master folder in your plugins folder.
  • Open QGIS and enable QChainage in the Plugin Manager
  • Open your line layer
  • Select the menu item Plugins->QChainage->Create Chainage
  • Fill in the options

    qchainage options

    I'm using 10 meters here. Distance is in map units.

  • Click OK

  • Profit...

    result

This will create a new point layer in memory if you would like to save it out Right Click the new point layer and select Save As...

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Thanks for the help Nathan. After some fruitless effort trying to install the plugin I read the post below from mach0.. :) I should have posted my software version with the question which is: QGIS 1.7.3-Wroclaw against code revision 00624b3. –  Bob Mitchell Jan 21 '13 at 17:01
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Welcome to the site. Since you mention Simple Line and Marker Line I'm going to assume that you're using ArcMap.

I'm not sure whether any of the line symbolisation methods lets you specify a distance which will be maintained as you zoom in/out.

A workaround may be to create a separate point layer, and place a point every 100 feet. See the help file under Creating new points along a line for instructions on how to do this. This could allow you to quickly count the points along a line to know how long it was.

Note that this method isn't "live" so you would need to re-create the points if any of the lines changed.

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Thanks Stephen, this is what I was trying to avoid however. –  Bob Mitchell Jan 21 '13 at 17:05
    
@BobMitchell If you do end up using this approach, you could script it using Python, to make it very easy to recreate the points when the underlying data changes –  Stephen Lead Jan 21 '13 at 21:13
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The only thing is that the plugin is currently working in master only .. because it uses function not provided in current stable release 1.8.. So you have to use the provided nightly code (probably in parallel to your existing stable installation) .. Usually I'd say that master is as stable as "stable 1.8" is - but there are a lot of restructuring going on to prepare master for a 2.0 release - so you might run into some problems .. Sorry for that - but QGIS is ongoing progress :)

As soon as there is a 2.0 there will be lots of news available..

Nathan - thanks for the algorithm and for propagating a "not now" released plugin - I think it's time to push it to the official repository ..

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you really should have added this as a comment to my question. Have a quick read of the FAQ gis.stackexchange.com/faq –  Nathan W Jan 7 '13 at 12:04
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