digitizingqgisvertices
I am trying to digitize in QGIS 3.14, and I have not been able to find an equivalent to the ArcGIS streaming tool. I was taught to digitize where (you trace the feature with your mouse and nodes are added automatically at a given interval). Otherwise you get weird blocky lines unless you zoom very close.
Not sure if I am looking for the right terminology, but can't seem to find the right tool in QGIS.
This is my current advanced digitizing toolbar
I was still using version 2.2.0 when I posted this question, I've since upgraded to higher versions, which now have the "advanced digitiser" built into the core QGis application.
The answer below where it mentions anything to do with adding the plugin from the repository, is now deprecated and should not be followed.
If your still using an older QGis then the repository plugin is the only way to go.
The usage of the tool built in and/or plugin version however is the same, so I've left the original answer as is, in order to serve as a mini tutorial on it's use.
Original Answer Follows
The solution for me was a plug-in called cadinput by Oliver Dlang, I'd installed this quite sometime ago, but never actually looked at what it did.
Then I stumbled across this documentation:
https://github.com/olivierdalang/CadInput
and a video about it on Vimeo.
Wondering where I could find the plugin, I looked in my plugins manager, only to find that I already had it installed, so I set about learning how to use it. I later found out, that from version 2.8.0 of QGis on-wards, this plug in is actually built into the core under the advanced digitizing tools.
The documentation doesn't really go into any detail, and there's no commentary on the video, essentially it all boils down to the following shortcut keys however:
Pick the feature you wish to digitize, and put your layer into editing mode as usual, in my case this is what I'm digitizing.
Once your in editing mode, click on your add new feature icon (again standard stuff) and then look for and activate either the advanced digitizing tools or the cad-input plugin (the icon is the same for both of them)
It looks like a set square with a ruler behind it.
A word of warning here though, if your using a newer QGis Don't Install the Cad-Input plugin. I found that with the plug-in installed, and a version of QGis that included the built in version, the short-cut keys stopped working.
Once I uninstalled the plug-in the shortcut keys started working again, and they are the key to using this tool effectively.
Once you activate the cad-input tool, you should find you have a panel that looks like the following:
(I've drawn some highlights on it...)
Looking at the shortcut-keys, C turns the construction mode button on/off, P toggles the parallel mode buttons, press it once to get perpendicular, press it twice to select parallel lines.
I'm not going to go fully into the parallel modes here, as the only one I was using was angle lock.
With the new feature tool active, and advanced digitizing enabled, draw the first two points of your rectangle, along one edge of the feature you wish to digitize.
You'll see once you get the second point clicked down, that a thin green construction line protrudes from the end of the line you created.
This is a construction line for the advanced digitizing tools. If you move your mouse downwards, to do the second face, you should find that your cad-input will start to show the angle your moving at.
It's at this point where there's a bit of a difference between the two versions.
In the built-in version, by default, the tool will automatically snap to 90 degree angles (Although you can change this in the options), if this happens you'll see the following:
Notice the blue line at a right angle to the bright green one?
In your digitizing panel, you'll see the angle should be at 90 degrees. If your using the plug-in version, you'll not see this blue line however, but you should still press ctrl+a to lock the angle in the tolls panel.
IN the plug-in version, if you press ctrl+a even with an angle that's not 90 degrees, you'll find that you can mouse over to the panel and type in 90 manually with out affecting your digitizing process, getting 90 in the box and locking it down, is what makes the rest of the process simple.
Continue to add points at the corners of your building, remembering to press ctrl+a after each point you add, this will ensure that your next point is always drawn at 90 degrees to the previous one.
Once you get to the last point, line it up by hand (Unfortunately the tool has no help here..) click to add the last point, then right click to close the polygon as normal.
There's a ton more functionality in the tools too, but no space to document it all here.
My final 100ft view however:
Wouldn't have been possible without it.
Is there a tool or equivalent algorithm for the logical fourth one: "Merge Parts"?
No, there isn't.
The adjacent parts of a multipolygon are considered a topological error and must be linked with the Fix geometries tool.
Is it even possible to select individual parts?
No, it isn't.
You can select features, not individual parts.
Is there anything else I'm missing?
I don't know. But you can refer to parts of collections with the geometry_n() function in expressions.
For example, you can create a new feature (in a separate layer) with the Geometry by expression tool, that contains the combination of two parts (or more if the function is nested) of a feature or of several, of any layer. If the parts are adjacent, they will form a single part in the new feature.
To create a feature formed by parts 1 and 2, of the feature_ID 0, of the 'multipolygons' layer:
combine(
geometry_n( geometry( get_feature_by_id( 'multipolygons', 0)), 1),
geometry_n( geometry( get_feature_by_id( 'multipolygons', 0)), 2)
)
Note than the Geometry by expression tool copies the table from the input layer. If the input layer have more than one features, more than one features will be created, but only one will have a valid geometry with that expression.
UPDATE:
About the WKT of the Multipolygon provided in the comments (https://pastebin.com/1ibYDzR5), I can't seem a line shared by both parts.
When loading the feature in QGIS, it can be seen (with an appropriate zoom level), that the parts are not adjacent.
Upper:
Lower:
Be sure to snapping when digitizing and editing (here is the manual). Otherwise, you can not merge those parts as they are in any way.
Best Answer
Enable "Advanced Digitizing Toolbar" and you can find "Stream Digitizing":