QGIS Digitizing

GIS and Sociological Geography » Syllabus » Finishing 2013 » What's that tool do? » QGIS Digitizing

Cut, Copy, Paste Features (YB)

Proin posuere sapien non nunc imperdiet dictum. Nulla non interdum justo. Etiam et lectus ante. Mauris sit amet neque ultrices eros porttitor aliquam. Ut ultricies aliquam odio eu adipiscing. Curabitur placerat condimentum risus, at commodo dolor sodales vitae. Fusce aliquam, sem vel eleifend consequat, diam orci gravida metus, ut aliquam erat dui vitae leo. Duis vulputate nisl sed arcu consequat, ac dignissim metus gravida

Add, Move, Delete Selected Feature(s) (HM)

The best way to explain adding, moving and deleting is to show you. Here we go:

Step one, add a new layer - you can also add features to an existing layer, but I think adding a new layer is better so that if you mess up you haven't modified your original layer.

Step two, what kind of vector do you want it to be? We're going to do a polygon in this tutorial, but you can also add a line vector or a point vector depending on what you want to add to your map. Note that you can't add line vector info (or point info) to a polygon layer.

Step three - toggle the edit button, if you don't have edit on you won't be able to add anything.

Step four - Draw your polygons (or lines, or points if you chose those vectors instead). Note that I have georeferenced a Mills Map from 1983 to be superimposed over a Mills vector I already had. I'm using the Add, Move and Delete features to add the vector information from the georeferenced map to my Mills vector. When you're done drawing your vector information (which you complete by right-clicking on a PC). A little pop-up will ask you to put in an id number - and it has to be a number. I like to just do 01, 02, etc. This will pretty much be the only information for these polygons in the attribute table until you add more info later.

Step five - Admire your shiny new polygons/lines/points.

Step Six - Move it around if you like - use the move tool to move the polygon you just created, or move a feature that already exists if it's wrong (or if you just need it out of the way while you draw something else).

Step Seven - Continue adding new features…Oh no, that was horrible, I did a terrible job there…

Step Eight - lets try fixing it using the Node Tool to move the node points around.

Step Nine - you know what, sometimes it's best just to delete it and re-draw it. Or maybe you want to delete vector information that is out-of-date or wrong. Either way, we're going to open the attribute table, and select the feature we want to get rid of - in this case, I called it "02". A little pop-up will ask if you're sure if you want to get rid of the selected feature, so hit ok.

And that about covers it. Now you know how to add, move and delete vector features.

Rotate Feature(s) (JK)

To rotate one or more features, first select the layer that contains the feature and make it editable by clicking on the “Toggle Editing” button, which appears with a small pencil image on the toolbar. Your layer and the individual features will appear surrounded by red marks.

Screen%20Shot%202013-12-07%20at%2011.18.32%20PM.png

Next, under the menu bar click “Edit” and then on the drop down menu select “Rotate Feature(s)”.

If you are editing a polygon layer, your cursor will appear as a small hand and you can select and drag the feature to rotate it.

Screen%20Shot%202013-12-07%20at%2011.20.37%20PM.png

While “Rotate Feature(s)” is selected, you can individually rotate as many features on that layer as you want.

After finishing rotating features, deselect the “Toggle Editing” and click “Save” when prompted to.

Simplify Feature (JN)

What is simplify feature?
This feature will allow you to reduce the number of vertices of a feature and not changing the geometry of the shape.

Why is this important?
The simplified features allows mappers to reduce the amount of nodes in complex polygons that have too many node. It is useful to simplify the polygons because it provides the mappers a simple intuitive interface for generalization, and allows mappers to see the effects would be before implementing them (QGIS manual, 2012). Simplifying too much is bad, but simplifying just enough to get the general shape allows mappers to visualize areas more cohesively.

Example:
I chose to simplify San Luis Opispo County, CA.

Follow these steps to simplify your shape:

1) Turn on "toggle editing" on the vector layer you are working on

1) Go to Edit > simplify feature

2) Choose the region you want to simplify (in my case it was San Luis Opispo County)

3) A slider will appear, and you will need to slide it to the left (less simplified- the original shape) and the right (simplified)

4) Save, and turn "toggle editing" off

5) VOILA! Done!

Add Ring (ZB)

Adding a ring means creating a hole inside of an existing vector polygon. I used a georeferenced raster of the Mills Campus to make the following example.

The existing polygon represents Mills campus area that doesn't affect my allergies severely.

By using the "Add Ring" tool, I was able to remove a part from the existing polygon. The removed part represents Mills campus area that severely affects my allergies (black acacia trees may be the culprit).

Add Part (Meagan T.)

The "Add Part" function allows you to add a polygon to an existing feature. In my example, I have selected one of many different polygons which are in the same layer. The "add part" polygon will become a part of the selected polygon. They now act as one feature.

In this before picture, one polygon in the polygon layer is selected, as shown with the yellow.

In the after picture, two polygons are selected but they are one single feature.

To use the "add part" function, use "select feature" to select the polygon onto which you want to add. Toggle editing on for that layer and find "add part" under Edit. Now your cursor will be able to click and indicate the boundaries of your added part. Right-click to finish the new section. Toggle editing off and save the layer. Now these two polygons will be a part of the same feature.

Delete Ring (JL)

The Definition: Delete ring is the opposite of Add Ring (seen above)! Deleting a ring means to erase an existing hole inside of an existing vector polygon (ex: filling the donut hole).

Example

I wanted to continue using the above example used for Add Ring, so I also used a georeferenced raster of the Mills Campus to demonstrate an example. This is were we left off.

Step One

Click on Settings>Options>Digitizing. Under Digitizing there is a section called snapping. In this section we will adjust the search radius for vertex edits. Change the 0.000 to 5, and select pixels from the drop down menu.

Step Two

Select the layer you want to delete the hole from and select Toggle Edit (layer>toggle edit). Red crosses will appear around the layer and vertex markers. Next, select Edit>Delete Ring and click on one of the hole vector markers and the hole will be deleted.

Viola! The Ring has been deleted!

Delete Part (KD)

Reshape Features (PT)

Reshape Features, similar to Add a New Feature, allows you to reshape your added line or polygon (features). In order to do so, you need to follow these steps:

Step 1: Open QGIS and open it with a vector file.
Step 2: Identify which version of QGIS you have since my tutorial is based on QGIS 2.0.1 Dufour! Yet, you should still be able to learn from this tutorial, it's just the matter of locating your Reshape Feature.
Step 3: Click on the Toggle Edit Button
Step 4: Look for your Reshape Feature on your tool bar or go to Edit to find
Reshape Feature.

Screen%20Shot%202013-12-06%20at%204.17.11%20PM.png

Step 5: Let's say you want to make the previous shape sharper with more edges. You will need to put the cursor and draw the shape that you want. Similar to how you Add a new Feature, you have to hold on to the Control button and click on the starting point to finish. If you did it correctly, your picture should look similar to this

Tutorial on youtube: http://www.youtube.com/watch?v=XgL2OTpBNaY

Offset Curves by AW

Offset tool: (Created around June 2012)

This tool can be used in both editing layers and background layers.
The definition of this tool is to create an offset from the given original (parent) curve.

Methods on creating an offset curve:
1) Click on NURBS object (NURBS = Non-uniform Rational B-Spline)
Definition: “A Mathematical model commonly used in computer graphics for generating and representing curves and surfaces. It offers great flexibility and precision for handling both analytic and modeled shapes.” - Wikipedia
2) Click onto the offset button
3) Click and drag on the curve where the offset is desired
4) After offset is created, congratulations!

New tool was released with version 1.8.0 of QGIS, called the Offset Curve
++++Official release note description:
Offset Curves – a new digitizing tool for creating offset curves was added.

The Offset Curves tool can be accessed through the Advanced Digitizing toolbar.
To access the toolbar go to View > Toolbars > Advanced Digitizing

The Offset Curve tool
1) This tool is used to create parallel shifts of line layers and polygon rings (single sided buffers with the geos function GEOSSingleSidedBuffer).
2) It can be applied to the edited layer (the geometries are modified) or also to the background layers (creates copies of the lines / rings and adds it to the edited layer).
3) This is ideally suited for the creation of distance line layers. The displacement is shown at the bottom of the taskbar.
4) You can also create a shift of a line layer if you go into editing mode and select the feature
5) It is also possible to make the Offset Curve tool active and drag the cross to the desired distance.
6) Any changes can be made with the Save Layer Edits tool

Split Features by CM

With this feature you can either split lines or polygons into two separate features. The icon, allows you to draw a line through the polygon or line that you want to split, and then creates the two separate features. One example is shown below of how a line drawn with the split feature then creates two separate features of the polygon.

Another example show how creating a line through a polygon, in this case North America, will then show a better thematic map of just the Gulf of Mexico.

Merge Selected Features (SC)

The merge selected features tool is an advanced digitizing tool that allows you to merge features that have common boundaries and the same attributes.

The example below shows how two separate polygon features are selected and merged to form one polygon.

Node Tool (AG)

As described in the user manual the node tool is used for the following:
“For shapefile-based layers as well as SpatialLite,PostgreSQL/PostGIS, MSSQL Spatial and Oracle Spatial tables the Node Tool provides manipulation capabilities of feature vertices similar to CAD programs. It is possible to simply select multiple vertices at once and to move, add or delete them altogether. The node tool also works with ‘on the fly’ projection turned on and supports the topological editing feature. This tool is, unlike other tools in QGIS, persistent, so when some operation is done, selection stays active for this feature and tool. If the node tool couldn’t find any features, a warning will be displayed.
Important is to set the property Settings ‣ Options ‣ Digitizing ‣ Search Radius: to a number greater than zero (i.e. 10). Otherwise QGIS will not be able to tell which vertex is being edited.”

What does this really mean?

Well you can manipulate your polygons, lines, and points manually. This is great if you are needing to change the location of a point to a different block. You can also change the shape and size of a polygon, however, this may not allow for an accurate change. The best part is that it is easy to do!

First step, select the layer you would like to change. For this example, I am going to change the location of a farmers market. You can see in the image that the farmers market layer is selected, and they are represent by the purple dots. Once your layer is selected, click toggle button so that you can edit your layer.

Then you should be able to select the “node” tool that is located near the toggle button.

Once this is highlighted you can click any feature on the layer and move the feature. I am going to move a farmers market that I think is too close to another market. Click once to select, and again to drag and move it. Once you are done, click the toggle button again and click save. You’ve just changed a map!

danryan.us