Mapping Toolbox 3.0

Creating a Half-Resolution Georeferenced Image

A referencing matrix allows you to specify how to spatially orient your data. The following example shows how to create a half-resolution version of a georeferenced TIFF image, using referencing matrices and Image Processing Toolbox™ functions ind2gray and imresize.

Contents

Step 1: Import a Georeferenced TIFF Image

Read an indexed-color TIFF image and convert it to grayscale. The size of the image is 2000-by-2000.

[X, cmap] = imread('concord_ortho_w.tif');
I_orig = ind2gray(X, cmap);

Read the corresponding worldfile. Each image pixel covers a one-meter square on the map.

R_orig = worldfileread('concord_ortho_w.tfw');

Choose a convenient format for displaying the result.

currentFormat = get(0,'format');
format bank
R_orig

R_orig =

             0         -1.00
          1.00             0
     206999.50     913000.50

Step 2: Resize the Image to Half Its Original Size

Halve the resolution, creating a smaller (1000-by-1000) image.

I_half = imresize(I_orig, size(I_orig)/2, 'bicubic');

Step 3: Construct a Referencing Matrix for the Resized Image

Find the map coordinates of the center of pixel (1,1) in the resized image: halfway between the centers of pixels (1,1) and (2,2) in the original image.

[x11_orig, y11_orig] = pix2map(R_orig, 1, 1)

x11_orig =

     207000.50


y11_orig =

     912999.50

[x22_orig, y22_orig] = pix2map(R_orig, 2, 2)

x22_orig =

     207001.50


y22_orig =

     912998.50

Average these to determine the center of pixel (1,1) in the new image.

x11_half = (x11_orig + x22_orig) / 2
y11_half = (y11_orig + y22_orig) / 2

x11_half =

     207001.00


y11_half =

     912999.00

Construct a referencing matrix for the new image, noting that its pixels are each two meters square.

R_half = makerefmat(x11_half, y11_half, 2, -2)

R_half =

             0         -2.00
          2.00             0
     206999.00     913001.00

Step 4: Visualize the Results

Display each image in map coordinates.

figure
h1 = mapshow(I_orig,R_orig);
ax1 = get(h1,'Parent');
set(ax1, 'XLim', [208000 208250], 'YLim', [911800 911950],'TickDir','out')

figure
h2 = mapshow(I_half,R_half);
ax2 = get(h2,'Parent');
set(ax2, 'XLim', [208000 208250], 'YLim', [911800 911950],'TickDir','out')

Mark the same map location on top of each image.

x = 208202.21;
y = 911862.70;
line(x, y, 'Parent', ax1, 'Marker', '+', 'MarkerEdgeColor', 'r');
line(x, y, 'Parent', ax2, 'Marker', '+', 'MarkerEdgeColor', 'r');

Graphically, they coincide, even though the same map location corresponds to two different pixel coordinates.

[row1, col1] = map2pix(R_orig, x, y)

row1 =

       1137.80


col1 =

       1202.71

[row2, col2] = map2pix(R_half, x, y)

format(currentFormat);

row2 =

        569.15


col2 =

        601.60

Credits

concord_ortho_w.tif, concord_ortho_w.tfw - derived from orthophoto tiles from:

  Office of Geographic and Environmental Information (MassGIS),
  Commonwealth of Massachusetts  Executive Office of Environmental Affairs
  http://www.state.ma.us/mgis

  For more information, run:

  >> type concord_ortho.txt