Example: Create an Area with Holes and Islands

In this example we open a new drawing and then we create an area object that uses branches to create "holes" in the area plus "islands" outside the area.   We will simply create the area with mouse clicks in the drawing.   



 As described in the Editing Drawings topic, we choose Create Area for the mode button in the main toolbar.  We click into the drawing to indicate the coordinates that will mark the area boundary.



When we mark the last coordinate we use a Shift-click to indicate this will be the end of the first branch.



We can then click to indicate the boundary of the first "hole" in the area, just as we did in the prior example.   When we reach the last coordinate in the boundary we Shift-click to indicate the end of that branch.



We then click again to indicate the coordinates for the boundary of the second "hole" in the area.  When we reach the last coordinate once again we Shift-click to indicate the end of that branch.



We now create "islands" outside of the area by clicking to mark the desired coordinates.   For the last coordinate of the first "island" we Shift-click to indicate the end of the branch.



We click again to add a series of coordinates for the boundary of the second "island."  For the last one we Click.



We can now right-click to call up the context menu for committing or abandoning changes.




We choose Save Changes in the context menu.   Instead of using the context menu, we could also press Ctrl-Enter to save changes using a keyboard shortcut.



The result is an area with two holes and two islands.  What appear to be three separate objects have through the use of branches been created as a single area object.



If we open the table for the drawing we see that only a single record has been created containing a single geom.   There is only one area object in the drawing.


Branches or Separate Areas?  - GIS practitioners tend to have varying opinions on when, if at all, it is a good idea to use branched areas to represent collections of islands.   Using a branched area to represent an area with holes in it is not a matter of choice, as that is the only way to create areas with holes in most GIS systems.   But using multiple branches to create areas with islands, that is, an object which visually looks like multiple area objects but instead is only a single object, may or may not be the right approach, depending on the data and how it is used.


We encounter such situations most often when working with data sets that represent countries or provinces which have many islands.   For example, If a drawing represents the countries of Europe the author of the data set may have felt it best to have one record per country with the boundaries of countries such as Greece or Croatia which have many islands that are part of the county being represented by branched area objects.


See Also



Editing Drawings




Editing Drawings


Info Pane


Example: Draw Lines, Areas and Points - Simple example of using basic mouse moves to add points, lines and areas to a drawing.


Example: Trace an Area in a Map over an Image Background - In a map with a drawing layer above an image layer, create an area object in the drawing by tracing over the outlines of something seen in the image layer below.


Example: Edit Coordinates While Creating an Object - When creating an object in a map using a tool such as Create Area, right in the middle of the process we can edit coordinates in the Info pane Coordinates tab.   This example shows the step by step process.


Example: Edit Attributes and Move a Point - We look at the attributes for a point in a drawing layer and edit one of the attributes using a more expanded Edit dialog.  We then move the point to a new location. Easy!


Example: Create a Line using the Info Pane - Step by step creation and modification of a line in a drawing using the Info pane Coordinates tab.


Example: Create an Area with a Hole - Create an area in a drawing where the area includes one or more holes.  This is similar to how we create areas that have islands as part of the area.   


Example: Create a Multipoint - How to create multipoints. This topic provides two examples:  First we create a multipoint and then next we create a multipoint having two branches.  The purpose of this topic is to help teach the implementation of geometry in Manifold and other typical spatial packages using a somewhat unusual and rarely met object type, the multipoint, which combines what appear to be many separate points into a single multipoint object.