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 not use the New Object dialog as in previous examples but will simply create the area with mouse clicks in the drawing.   

 

eg_newobj03_01.png

 

We choose the Create Area tool and then we begin to click into the drawing to indicate the coordinates that will mark the area boundary.

 

eg_newobj04_01.png

 

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

 

eg_newobj04_02.png

 

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.

 

eg_newobj04_03.png

 

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.

 

eg_newobj04_04.png

 

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.

 

eg_newobj04_05.png

 

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

 

eg_newobj04_06.png

 

We can now right-click and choose Save Changes to create the area object.

 

eg_newobj04_07.png

 

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.

 

eg_newobj04_08.png

 

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.

 

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 is sometimes considered a controversial practise.

 

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

Drawings

 

Coordinates

 

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.

 

New Object Dialog

 

Example: Create a Line using the New Object Dialog - Step by step creation of a line in a drawing using the New Object Dialog.

 

Example: Create a Line using Curved Segments - Creating a line made up from curvilinear geometry using the New Object Dialog.

 

Example: Create an Area with a Hole - Using the New Object Dialog, 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 using the New Object Dialog.  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.