Mapping Toolbox

Simplifying Vector Data

Sometimes you may need to reduce the number of points in your vector data, while still maintaining an accurate representation of the geographical data. A quick and easy method would be to throw out every other element or set of elements in each vector. However, the result may be a very crude representation of the original data. Another option exists. The Mapping Toolbox provides a function implementing a powerful line simplification algorithm that selectively deletes points.

Note    Reduced geographic data may not always be appropriate for display. If all intermediate points in a dataset are reduced, then lines appearing straight in one projection will be incorrectly displayed as straight lines in others.

For example, reduce the number of points in the state of Massachusetts with the reducem function:

• load usahi
  whos
  
  

  Name	Size	Bytes	Class
  stateline	1x51	837656	struct array
  statepatch	1x51	837758	struct array
  statetext	1x51	51190	struct array

The stateline structure contains vector data for all 50 U.S. states and the District of Columbia. We are only interested in the state of Massachusetts, the 19th entry in the structure:

• lat = stateline(19).lat;
  long = stateline(19).long;
  clear state*
  

Now simplify your vector data:

• [newlat,newlong,cerr,tol] = reducem(lat,long);
  whos
  
  

  Name	Size	Bytes	Class
  cerr	1x1	8	double array
  lat	958x1	7664	double array
  long	958x1	7664	double array
  newlat	253x1	2024	double array
  newlong	253x1	2024	double array
  tol	1x1	8	double array

The Massachusetts vector data has been reduced to almost a quarter of its original size:

• size(newlat)/size(lat)
  ans =
      0.2641
  

Plots of the datasets, however, show two virtually identical maps.

Trimming Vector Data to a Defined Region

Working with Matrix Maps