[Qgis-user] Record the cracks in our (private) streets

[Nicolas Cadieux]

Hi Doug 
I guess you are new to QGIS and GIS in general? See below.

> Le 14 déc. 2019 à 01:05, Doug <dougf.ccn at comcast.net> a écrit :
> 
> 
> I have imported a set of drone pictures to QGIS. They show the streets in my subdivision. I want to “record” the cracks in the pavement to be able to compare  them over time. I want to be able to segment the streets in arbitrary lengths. For each segment, I want to “draw” the cracks (in layers) to record where they are. Every year or two I will repeat the process. The purpose is to be able to compare the set of cracks in successive snapshots to see how the asphalt is holding up and identify underlying problems. I would love to have some software to find the cracks for me and generate the crack layer, if anyone knows of such software.

How many streets are we looking at?  You can use a QGIS plugin like the QGIS  semi-automatic classification plugin to help you identify the cracks.  You can also explore raster change Detection algorithms. Between you and me through, the time you will spend calibrating each image for these methods to work will probably be much longer than the time spent doing it visually. If you try an automatic method and fail, you will still learn a lot.

>  
> First problem: I can trace the outline of the streets to generate a layer which represents to road surface. Is there a way to slice that layer by picking two points to “slice” the layer and then have two layers? And then repeat the process so that I get the segments that I want? Alternatively, is there a way to create a segment and “snap” to next segment so there is not overlap or missing space?

You can clip a raster file(the images) with vector Polygons. Google “clip raster” and look in the QGIS documentation. If everything is georeferenced, raster layer will be correctly placed if you open the different file.  It’s better to have a local utm based CRS (Coordinates Référence system).  This may depend on the drone software you are using. It may be in WGS84 long lat by default.  You can change the CRS of the images by reprojecting it but don’t go back and forth.  Keep a backup of the originals.  You can change the crs of the project thought without problem.  You can use a plugin like the quickmapservices to import raster like google earth to make sure things are looking good for the georeferencing.  

https://qgis.org/en/docs/index.html

>  
> Second problem. Given that I have a layer representing a segment of the road surface, can I “draw” the cracks for that segment such that every line in the cracks layer is related (as in SQL) to the segment it reside in?
>  

Yes, vector file formats like shapefile or SpatiaLite will link any line, point or polylines that you draw to a database table.  In that table, you can add information manually, like the street name, or you can generate informations like line length based on the drawing using expressions in he field calculator.  You can link a crack to a street address if you have the georeferenced street addresses in another file. Google spatial queries.

https://docs.qgis.org/3.4/en/docs/user_manual/working_with_vector/index.html

> Once I have the road surface segments, and the associated cracks, I can then use SQL to generate all sorts of reports on the cracks: average and median length, number of horizontal and vertical cracks, total crack length per road surface segment (or square feet), etc.
>  

You can use QGIS to generate reports (look in various plugins also) or any favorite tools like excel if you export the table fields to a csv.

Good luck with the project.
Nicolas

> Thanks.
> Doug
>  
>  
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