Is there a way to add a buffer zone to gis:set-world-envelope-ds gis:envelope-of *shapefile*

Question

Currently,

I am using gis:set-world-envelope-ds gis:envelope-of <shapefile> to set world. However, I am getting the error message: " Cannot move turtle beyond the world's edge. error while node 18469 running SET"

Hence, I would like to add a bufferzone around it. Does anyone know how?

Answer 1

You can manually adjust the envelope values, since they are x/y bounds. Calling gis:envelope-of just returns a list of the format [x1 x2 y1 y2] which represent the boundaries of the envelope. How to adjust those depends on how much boundary you want and your projection but: here is a quick and dirty way to add a tenth of a 'span' (x1 - x2 or y1 - y2) as a buffer:

extensions [ gis ]

to setup
  ca
  let shp_path "C:/gis_example/british_columbia_administrative.shp"
  let prj_path "C:/gis_example/british_columbia_administrative.prj"
  
  gis:load-coordinate-system prj_path
  let shp gis:load-dataset shp_path
  let base_envelope gis:envelope-of shp
  
  print word "Base envelope: " base_envelope
  
  let span_x abs ( item 0 base_envelope - item 1 base_envelope )
  let span_y abs ( item 2 base_envelope - item 3 base_envelope )
  let alternating_spans ( list span_x span_x span_y span_y )
  let alternating_signs [ -1 1 -1 1 ]
  
  let expanded_envelope ( 
    map [ [ i span sign ] -> i + sign * ( 0.1 * span )  ] 
    base_envelope alternating_spans alternating_signs 
  ) 
  
  print word "Expanded envelope: " expanded_envelope
  gis:set-world-envelope-ds expanded_envelope  
  gis:set-drawing-color white
  gis:draw shp 1  
  
  reset-ticks
end

Dataset downloaded from MapCruzin.com