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I have read several tutorials and questions here on this site, but I have some lingering questions/doubts that I would like clarified.

My project is tracing roadways from a 1926 map so that I can compare infrastructure then and now. I have successfully georeferenced my 1926 raster map onto a contemporary shapefile, reduced transparency, etc. I have enabled freehand line editing, which is the best way to accomplish this task, it seems. My map has two kinds of roads shown that I want to trace: the paved roads, and the unpaved roads (marked by full, bold lines and staggered, broken lines). So, then, here is what I would like clarified.

  1. In the New Vector Layer box, an option is given for Name, Type, Width... I have named the feature line 'roads,' which is relevant for question #2. What exactly is the Type option (text data, whole number, decimal number, etc.) for, what is it not for, and could someone give a quick example of when this option might be useful?
  2. When I trace with the freehand edit tool, I realize that when I unclick, several points/nodes/features (whatever they are called!) are placed along the traced line as if I clicked them each individually. OK... so there appears a popup box asking me for 'id' and 'roads'--'id' is blank, and 'roads' is 'NULL.' What exactly is going on here with this? So far, I have simply left the box as-is and clicked 'OK.' Would I regret this later if I wanted to edit the lines or something? What is the point of this box, the information in it, and how is it useful?
  3. The paved roads I want to trace are not connected to each other--some paved roads in the north, south, etc. So far, what I have been doing is tracing a line of paved roads as described above, and then moving my location in the map to do the same in the north or other areas. Following this procedure as described, will all the lines traced be counted/recognized as the same feature/network of feature? Is it alright to trace a road coming into a city from one direction, unclick, and then trace the road coming into the city from the other direction, and will it still be part of the same road?
  4. How do I trace the unpaved roads once I get the paved roads all traced? (I suspect that I would need to allow for this when creating the new shapefile layer from the start, by adding a 'roads' AND another thing like 'unpaved roads,' is this correct? But if I have not done this from the beginning, am I screwed and should start over?)

closed as too broad by PolyGeo Feb 28 '17 at 6:26

Please edit the question to limit it to a specific problem with enough detail to identify an adequate answer. Avoid asking multiple distinct questions at once. See the How to Ask page for help clarifying this question. If this question can be reworded to fit the rules in the help center, please edit the question.

  • As per the Tour there should be only one question asked per question – PolyGeo Feb 28 '17 at 6:27
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In the New Layer Box, you can create a data field for Road numbers (if they are printed on the map), and another field named "paved", where you can indicate whether a road is paved (yes or 1) or unpaved (no or 0). If your map contains street names, you can set up a field for that too. Text fields would be best for the beginning.

After digitizing a line, you can leave the id field empty (it can be automatically set by QGIS if necessary) end enter the values for the other fileds.

I do digitizing with the normal editing tools.

If you want the roads to be connected, enable snapping with a radius of 10 pixels. Then the cursor will jump to a point already set with a previous line nearby.

What belongs to the same road is a question of definition. Do you want all parts of the same street name, or road numer, or what?

  • Ahh, good point. What I really want is to be able to group all the paved roads together for future analysis and all the unpaved together, so that is what I meant by the same road. Bit misleading, apologies. – grad student Dec 19 '13 at 15:53
  • You can get that with a category field "paved". In Settings -> Options -> Digitizing, you can choose to preset the form with the data from the last object. That will save you much time if you start with all the paved roads. – AndreJ Dec 19 '13 at 16:57
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1) Those options set what type of data is stored in that field, either text, which you can limit to a certain number of characters, and either numbers stored as integers or decimal.

2) The box that pops up after you finish a feature allows you to populate the attribute table for that feature.

3) As is it doesn't really matter which way you digitize, shapefiles have no concept of a network. If you move your data into another format to allow routing you'll have to take additional steps to limit roads to one way, indicate direction, etc.

4) You can either have two separate shapefiles representing paved and unpaved roads, but I prefer to have one road shapefile with a field in the attribute table that indicates paved or unpaved. Then in the layer's symbology I symbolize by category, and then my paved roads can be one color and unpaved another.

  • For #1, what would be the value of having paved and unpaved roads as text, numbers, etc. What does this really mean? For #2, it appears I cannot type in anything in the 'id' field but I can alter what is in the 'roads' field. What's going on with this? For #3, so regardless of whether I am tracing lines here and there, in the process I have described, the roads will be grouped as one road? – grad student Dec 19 '13 at 15:42
  • For #1 it depends on how you want to handle your data, and that data is the real power of GIS, without it you just have lines on a map. Those fields in the background are where you can begin to do some real analysis. You can add as many fields as you need to track data related to those roads, or to allow you to tie your road layer to other tables in other databases and spreadsheets. The datatype (text, number, etc) are how the computer handles the data in that field. You might want to look at some tutorials on databases and creating tables to get a better idea of what's going on there. – HeyOverThere Dec 19 '13 at 16:00
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    For #2, I'm not sure why you can't fill in the ID field. It may be autopopulated for you. For #3, as is most likely each individual feature will be standing alone. You can group them later, into a multiline string. Look up multilines and multipolygons to figure out if they will work for what you're trying to do. – HeyOverThere Dec 19 '13 at 16:01

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