I want to extract contour from scanned map, but that map is scattered very much, so it is impossible to extract automatically.
So I make line shapefile and try to use tracing tool
but I cannot do that because my map is just image.
How can I use tracing tool for my map?
The AutoTrace is a plugin for QGIS that allows users to digitize new features based on vertices from existing features. It essentially allows you to trace existing features. You cannot use it to trace an image directly, it use to with existing vector layer to make polygons contiguous.
The only way to trace contour from an image is to do it manually by creating new line feature and start digitizing which I know it is time consuming, but it is more accurate and faster than trying to convert raster to vector automatically and then cleaning that vector layer.
Another option is to use a DEM and extract contour from the DEM but it depends on the scale that you are looking for. If contour image covering detailed area (large scale), then only digitizing is the option. However, if you are working with small scale, you can use DEM to extract contour from it.
You can try using raster_tracer plugin for QGis.
https://github.com/mkondratyev85/raster_tracer
RasterTracer is a plugin for semi-automatic digitizing of underlying raster layer in QGis. It is useful, for example, when you need to digitize a scanned topographic map, with curved black lines representing lines of equal heights of the surface. Instead of creating this curved vector line by manually clicking at each segment of this curved line to create multi-line, with this plugin you can click at the beginning of the curved line and at the end of the curved line, and it will automatically trace over black pixels (or pixels that are almost black) from the beginning to the end. By using this plugin you reduce clicks while digitizing raster maps.
