Ground control point targets are useful to stitch photos on uniform or repeatitive landscapes. They can be made from cardbord plates, cloth etc. See for example this note.

However, on very uniform areas, even ground control points may not help: photos of green patches of grass with identical control points are hard to tell apart. In such situations, a coded ground control point marker comes in handy.

A simple approach I thought up over coffee with Thomas Groen is to make a paper sheet with a 3x3 grid of black-or-white squares, forming a binary code of 9 positions. The codes in the photo above for example are numbers 000.000.001 and 000.000.101 (when white = 1, black = 0).

Paper sheets with codes can be generated easily with some easy code, such as this script for Processing:

#7E7E7E;">// Coded ground control points //

#CC6600;">import processing.pdf.*;

#CC6600;">size (297, 420,PDF, #006699;">"GCPs.pdf"); #7E7E7E;">// setup for A3 standard sheets
#7E7E7E;">// load font'
#CC6600;">PFont font;
font = #CC6600;">loadFont(#006699;">"Georgia-12.vlw");

#CC6600;">int achtergrond = 200; #7E7E7E;">// sets background

#CC6600;">for (#CC6600;">int teller = 0; teller <= 98; teller = teller +1) { 

  #CC6600;">background(achtergrond);
  #CC6600;">String #CC6600;">str = #CC6600;">binary(teller, 9);
  #CC6600;">println(#CC6600;">str);
  #CC6600;">for (#CC6600;">int i = 0; i < 3; i = i+1) {
    #CC6600;">for (#CC6600;">int j = 0; j < 3; j = j+1) {
      #CC6600;">stroke(achtergrond);
      #CC6600;">fill(achtergrond*(#CC6600;">int(#CC6600;">str.#CC6600;">charAt(j+i*3))-48));
      #CC6600;">rect(4+96*j, 4+96*i, 95, 95);
    }
  }
  #7E7E7E;">// draw scalebar of 250 mm long
  #CC6600;">fill(1, 1, 1);
  #CC6600;">stroke(1);
  #CC6600;">rect(23, 308, 250, 94);

  #7E7E7E;">// draw cross for GPS capture"
  #CC6600;">stroke (achtergrond);
  #CC6600;">strokeWeight(2);
  #CC6600;">line(149-20,354,149+20,354);
  #CC6600;">line (149, 354-20,149, 354+20);

  #7E7E7E;">// add text 
  #CC6600;">fill(100);
  #CC6600;">textFont(font, 6);
  #CC6600;">text(#006699;">"jasjavliegt.nl ", 40,330);
  #CC6600;">text(#006699;">"Nr. "+teller, 40,350);
PGraphicsPDF pdf = (PGraphicsPDF) g;  #7E7E7E;">// Get the renderer
 pdf.nextPage();  #7E7E7E;">// Tell it to go to the next page
}
#CC6600;">exit();