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					About georeferencing
Raster data is commonly obtained by scanning maps or collecting         The number of links you need to create depends on the
aerial photographs and satellite images. Scanned map datasets           complexity of the polynomial transformation you plan to use to
don’t normally contain spatial reference information, either            transform the raster dataset to map coordinates. However, adding
embedded in the file or as a separate file. With aerial photography     more links will not necessarily yield a better registration. If
and satellite imagery, sometimes the locational information             possible, you should spread out the links over the entire raster
delivered with them is inadequate, and the data does not align          dataset rather than concentrating them in one area. Typically,
properly with other data you may have. Thus, in order to use            having at least one link near each corner of the raster dataset and
some raster datasets in conjunction with your other spatial data,       a few throughout the interior produces the best results.
you often need to align it, or georeference it, to a map coordinate     Generally, the greater the overlap between the raster dataset and
system. A map coordinate system is defined using a map                  target data, the better the alignment results, because you’ll have
projection (a method by which the curved surface of the earth is        more widely spaced points with which to georeference the raster
portrayed on a flat surface).                                           dataset. For example, if your target data only occupies one
When you georeference your raster dataset, you define its               quarter of the area of your raster dataset, the points you could
location using map coordinates and assign a coordinate system.          use to align the raster dataset would be confined to that area of
Georeferencing raster data allows it to be viewed, queried, and         overlap. Thus, the areas outside the overlap area are likely not
analyzed with other geographic data.                                    properly aligned.
                                                                        Keep in mind that your georeferenced data is only as accurate as
How to align the raster dataset
                                                                        the data to which it was aligned. To minimize errors, you should
Generally, you will georeference your raster dataset using existing     georeference to data that is at the highest resolution and largest
spatial data (target data), such as a vector feature class, that        scale for your needs.
resides in the desired map coordinate system. This assumes that
there are features in your spatial data that are also visible in the    Transforming the raster dataset
raster—for example, street intersections or building corners.           When you’ve created enough links, you can transform—or
The process involves identifying a series of ground control             warp—the raster dataset to permanently match the map
points—known x,y coordinates—that link locations on the raster          coordinates of the target data. Warping uses a polynomial
dataset with locations in the spatially referenced data (target         transformation to determine the correct map coordinate location
data). The control points are used to build a polynomial                for each cell in the raster.
transformation that will convert the raster dataset from its existing   Use a first-order—or affine—transformation to shift, scale, and
location to the spatially correct location. The connection between      rotate your raster dataset. This generally results in straight lines
one control point on the raster dataset (the “from point”) and the      on the raster dataset mapped as straight lines in the warped raster
corresponding control point on the aligned target data (the “to         dataset. Thus, squares and rectangles on the raster dataset are
point”) is called a link.                                               commonly changed into parallelograms of arbitrary scaling and
                                                                        angle orientation.

WORKING WITH RASTERS                                                                                                                    305
A first-order transformation will probably handle most of your          Resampling the raster dataset
georeferencing requirements. With a minimum of three links, the
                                                                        While you might think each cell in a raster dataset is transformed
mathematical equation used with a first-order transformation can
                                                                        to its new map coordinate location, the process actually works in
exactly map each raster point to the target location. Any more
                                                                        reverse. During georeferencing, a matrix of “empty” cells is
than three links introduces errors, or residuals, that are
                                                                        computed using the map coordinates. Then, each empty cell is
distributed throughout all the links. However, you should add           given a value based on a process called resampling.
more than three links because if one link is positionally wrong, it
has a much greater impact on the transformation. Thus, even             The three most common resampling techniques are nearest
though the mathematical transformation error may increase as            neighbor assignment, bilinear interpolation, and cubic
you create more links, the overall accuracy of the transformation       convolution. These techniques assign a value to each empty cell
will increase as well.                                                  by examining the cells in the ungeoreferenced raster dataset.
                                                                        Nearest neighbor assignment takes the value from the cell closest
The higher the transformation order, the more complex the               to the transformed cell as the new value. It’s the fastest
distortion that can be corrected. However, transformations higher       resampling technique and is appropriate for categorical or
than third order are rarely needed. Higher-order transformations        thematic data. Bilinear interpolation and cubic convolution
require more links and thus will involve progressively more             techniques combine a greater number of nearby cells (4 and 16,
processing time. In general, if your raster dataset needs to be         respectively) to compute the value for the transformed cell. These
stretched, scaled, and rotated, use a first-order transformation. If,   two techniques use a weighted averaging method to compute the
however, the raster dataset must be bent or curved, use a second-       output transformed cell value and thus are only appropriate for
or third-order transformation.                                          continuous data, such as elevation, slope, aerial photography,
                                                                        and other continuous surfaces.
Interpreting the root mean square error
                                                                        Resampling is part of the georeferencing process; however, it can
The degree to which the transformation can accurately map all           also be used to adjust the cell size of a raster dataset or can be
control points can be measured mathematically by comparing the          part of a geoprocessing operation to alter the values of cells.
actual location of the map coordinate to the transformed position
in the raster. The distance between these two points is known as        Should you rectify your raster dataset?
the residual error. The total error is computed by taking the root      You can permanently transform your raster dataset after
mean square (RMS) sum of all the residuals to compute the RMS           georeferencing it by using the Rectify command on the
error. This value describes how consistent the transformation is        Georeferencing toolbar. Rectify creates a new raster dataset that
between the different control points (links). Links can be removed      is georeferenced to map coordinates. You can save this in ESRI
if the error is particularly large, and more points can be added.       GRID, TIFF, or ERDAS® IMAGINE® format.
While the RMS error is a good assessment of the accuracy of the         ArcMap doesn’t require you to rectify your raster dataset to
transformation, don’t confuse a low RMS error with an accurate          display it with other spatial data. You might choose to rectify
registration. For example, the transformation may still contain         your raster dataset if you plan to perform analysis with it or want
significant errors due to a poorly entered control point.               to use it with another software package that doesn’t recognize
                                                                        the external georeferencing information created by ArcMap.

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The Georeferencing toolbar
                             Set the raster dataset to
                             georeference.                              Add control points (links).

                                                                                             View the link table.
              Save the
  transformation with                                                                       Rotate or shift the
    the raster dataset.                 Shift the raster                                    raster dataset before
                                        dataset to the current                              adding links.
                                        display area.
       Create a new,
  transformed raster                                                                        Correct for common
            dataset.                                                                        scanning distortions.

   Adjust the display
    as you add links.

                                                                 Set the transformation order.

WORKING WITH RASTERS                                                                                            307
Georeferencing a                   Georeferencing a raster
raster dataset
                                   1. Add the layers residing in
The general steps for                 map coordinates and the                                                             3           5
georeferencing a raster dataset       raster dataset you want to                                        4
are:                                  georeference.
 1. Add the raster dataset that    2. In the table of contents, right-
    you want to align with your       click a target layer—the                                           When enabled, the raster
    projected data.                   referenced dataset—and                                             dataset automatically shifts as
                                      click Zoom to Layer.                                               you enter each link.
 2. Add control points that link
    known raster dataset           3. From the Georeferencing
    positions to known posi-          toolbar, click the Layer
    tions in map coordinates.         dropdown arrow and click
 3. Save the georeferencing           the raster layer you want to
    information when you’re           georeference.
    satisfied with the             4. Click Georeferencing and
    registration.                     click Fit To Display.

                                      This will display the raster
                                      dataset in the same area as
                                      the target layers. You can
Displaying the
                                      also use the Shift and Rotate
Georeferencing toolbar
                                      tools to move the raster
Right-click the Tools menu and
                                      dataset as needed. To see all
point to Georeferencing.
                                      the datasets, you may have
                                      to adjust their order in the
                                      table of contents.
                                   5. Click the Control Points tool
                                      to add control points.             To create a link, click a control point on the raster dataset,
                                                                         then click the corresponding control point on the target data.
                                   6. To add a link, click the mouse
                                      pointer on a known location
                                      on the raster dataset, then on
                                      a known location on the data
                                      in map coordinates—the
                                      referenced data. u

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Tip                                       You may find it useful to use
                                          a Magnification window to
What features could I use
                                          add in your links. When
as control points?
You could look for road intersec-
                                          working with two raster
                                          datasets, you may want to
                                                                                                              9                                8
tions, land features, building
                                          open the Effects toolbar and
corners, or other objects that you
                                          adjust the transparency or
can identify and match in your
raster dataset and aligned datasets.      turn layers on and off in the
                                          table of contents to view
                                          each image as you add your
Tip                                       links.
Deleting a link                        7. Add enough links for the
You can delete an unwanted link           transformation order. You
from the Link Table dialog box.           need a minimum of three
Press the Esc key to remove a link        links for a first-order transfor-
while you’re in the middle of             mation, six links for a second
creating it.                              order, and 10 links for a third
Tip                                    8. Click View Link Table to
Using the georeferencing                  evaluate the transformation.          Evaluate the links.
functions                                 You can examine the
The Rotate and Shift tools are no         residual error for each link
longer available after you place the      and the RMS error. If you’re
first link.
                                          satisfied with the registration,
                                          you can stop entering links.
Tip                                    9. Click Georeferencing and
Transforming the raster                   click Update Georeferencing
dataset permanently                       to save the transformation
You can permanently transform             information with the raster
your raster dataset after                 dataset.
georeferencing by using the Rectify
                                          This creates a new file with
command. Click Georeferencing
                                          the same name as the raster
and click Rectify.
                                          dataset, but with an .aux file
                                          extension. It also creates a
                                          world file for .tif and .img files.

                                                                                After updating georeferencing information, the raster dataset will align
                                                                                to other spatial data when added to a map.

WORKING WITH RASTERS                                                                                                                                 309
Tip                                     Entering explicit x,y map
What is my georeferenced                coordinates
raster dataset’s coordinate
                                        1. Click the View Link Table
                                           button on the Georeferencing                      21
The coordinate system assigned to
the raster dataset is the same as the
                                           toolbar.                             3
coordinate system defined for the       2. Click the Control Points tool.
data frame.                             3. Click the mouse on a known
                                           location in the unreferenced
Tip                                        image to add the first
                                           coordinate in the link.                      4
Adding explicit values
while georeferencing                    4. Right-click the image and
You can add explicit values after          choose Input X and Y.
you have clicked a control point on     5. Enter the reference coordi-
your raster dataset. Just right-click
                                           nates in the Enter Coordi-
on your map and choose Input X
                                           nates dialog box.
and Y.
                                        6. Click OK.



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