Digitizing PNG image in QGIS

Question

Following Getting polygons from PNG images for the QGIS Shapefiles

I intended to digitize the following PNG image in QGIS 3.8 following this instruction:

https://www.qgistutorials.com/en/docs/digitizing_basics.html

which is valid for version 2.2 (2+)

In the result some steps don't match to the 3.8 version, therefore I have a few doubts/problems, that I already encountered:

1. I cannot manage with snapping options, as error occur:

2. The edit toolbar doesn't match with the tutorial for 2.2 I should have an option to drawing the polyline and polygon, which doesn't appear here. In turn I can only draw the straight line from the topleft corner of the map.

3. Settings don't provide the "Snapping options"

4. I have no option for drawing (redrawing) the existing lines on the PDF image

Is anyone able to give me some hint?

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Even if I activate the Snapping toolbar I am still unable to draw the lines. I can only mark the polygons as per in the image below:

Answer 1

Looks like you have a 'line type' of layer so I don't think you can draw polygon in it, only lines.

You can add the snapping toolbar by right clicking on around the top right of the QGIS window (next to all the icons) and activate it.

Answer 2

Here are a few tips and pieces of advice that should help you:

• Snapping only works to snap to vector layers. You're trying to digitize a line from a raster image. QGIS can't snap to the line in the image because it's raster data, not vector data. You may look at the image and see a line with vertices and segments, but QGIS only sees pixels. So you will have to do the digitizing manually.

• The point of using the snapping tool in your second tutorial, is to snap features together as you create them. After you already have one line, the second line (or polygon) can then snap to the first line, which makes sure you don't have any gaps of overlaps.

• As Mr_Yum pointed out, most vector formats can only store one type of geometry per vector layer. So if you want to digitize lines and polygons, you need two separate vector layers (one polygon layer and one line layer).

• The pdf looks like it was originally created using vector data. If you can obtain the original vector data, you won't have to digitize it from the pdf. I can't tell what type of information those polygons are, but it's possible that that data is publicly available. If you think the original data might be publicly available somewhere (but you don't know where), try asking on Open Data Stackexchange for help finding it.

• If you have a touchscreen, you might find the Freehand Editing plugin makes it easier to digitize the features. This plugin lets you digitize a feature by drawing it with a stylus instead of clicking for each vertex.

• Reverting to an older version of QGIS. No version of QGIS has the ability to extract this type of vector features out of a raster image like the one you have. You will always have to do it manually.