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What is
a digital orthophoto?
Digital orthophotos are photographic images that combine the
image characteristics of an aerial photograph with the geometric
qualities of a map. Ordinary aerial photographs have inconsistencies
in scale due to variations in terrain elevation, changes in
distance from the camera to the terrain across the field-of-view,
and tilting of the camera itself when the plane is not flying
perfectly straight and level. To create an orthophoto,
an aerial photograph is scanned using a precise, high-resolution
scanner. Each image pixel is then processed through photogrammetric
equations using ground control points, camera calibration and
orientation parameters, and a digital elevation model.
The result is an orthorectified image in which distortions and
displacements are removed, allowing distances, areas, and angles
to be precisely measured.
TeraServerUSA orthophotos have a ground resolution of 1 meter
(i.e. each pixel represents 1 m2) at highest resolution
and a nominal scale of 1:12,000. The images use
a Universal Transverse Mercator (UTM) grid based on the North
American Datum of 1983 (NAD83).
What
is a world coordinate file?
A world file is a small text file that accompanies some image
file formats such as JPG, PNG, BMP and TIFF. The corresponding
world file for such images would be given JGW, PGW, BPW or TFW
file extensions. The first part of an image and corresponding
world file must have the same name (such as mountain.jpg and
mountain.jgw) and be located in the same folder.
The world file provides information that can be used to register
an image to real-world coordinates. It contains parameters
in plain text which can be used to establish an image-to-world
transformation that converts the image coordinates to real-world
coordinates. Some GIS programs will automatically use
this information to register an image when you display it.
The contents of the world file might look something like this:
| 1.282090 |
| 0.000000 |
| 0.000000 |
| -1.282090 |
| 730691.000000 |
| 4797771.000000 |
|
World file parameters
are always stored in this order:
|
| A. |
X-scale (meters per pixel in the X direction) |
1.282090
|
| D. |
Rotation in X direction (assumed = 0) |
0.000000
|
| B. |
Rotation in Y direction (assumed = 0) |
0.000000
|
| E. |
Negative of Y-scale (meters per pixel in the Y direction) |
-1.282090
|
| C. |
Easting
Coordinate of the center of the upper left pixel of
the image |
730691.000000
|
| F. |
Northing
Coordinate of the center of the upper left pixel of
the image |
4797771.000000
|
Note that simple world files contain no information
about the type of projection or coordinate system and datum
used to calibrate the image. You'll need to supply that information
(often found in metadata text files from distributors of geographic
data) if your spatial mapping program requests it. Some programs
like fGIS will not require a name for the projection or coordinate
system and datum, but will require all data to be based on a
similar system in order for layers to line up. FYI, GeoTiff
images have headers that contain information similar to world
files. GeoTiff headers may also have flags for additional information
like projection, datum, spheroid, etc.
If you want to create a world file for an image, see the Digital
Gove tutorial for using the TatukGIS
Viewer and HyperCube (two freeware programs) for image calibration
and rectification.
The world file values can be used in a six-parameter affine
transformation in the form of:
x1 = Ax + By + C
y1 = Dx + Ey + F
where
x1 = calculated x-coordinate
of the pixel on the map
y1 = calculated y-coordinate of the pixel on the map
x = column number of a pixel in the image
y = row number of a pixel in the image
A = x-scale; dimension of a pixel in map units
in x direction
B,D = rotation terms (assumed to be zero)
C,F = translation terms; x,y map coordinates of the center
of the upper-left pixel
E = negative of y-scale; dimension of a pixel in
map units
in y direction
Affine transformations involve rotating, scaling and skewing
an image in a manner that maintains the relative position of
points within the image (e.g., parallel lines stay parallel).
Note: The y-scale (E) is negative because the
origins of an image and a geographic coordinate system are different.
The origin of an image is located in the upper-left corner,
whereas the origin of the map coordinate system is located in
the lower-left corner. Row values in the image increase from
the origin downward, while y-coordinate values in the map increase
from the origin upward.
What
is a Map Projection?
The curved, three-dimensional surface
of the earth is difficult to represent on a flat, two-dimensional
map. A map projection defines the spatial relationship
between features on the earth's surface (3D) and their representations
on a map (2D). It is a mathematical expression based on a sphere
or spheroid, (conic, cylindrical, or planar) which transforms
the earth's curved terrain to a flat surface. Map projections
cause distortion to one or more map properties, such as scale,
shape, area, distance, or direction. Hundreds of map projections
have been developed to accurately represent at least one map
property. A map projection is selected based on
scale and which map property must be preserved.
What
are Coordinate or Grid Systems?
A coordinate system is a reference
grid used to locate the position of features on a map.
It is a two-dimensional grid displayed as rows and columns with
each presenting a unit of distance. Out of convenience, UTM
and Geographic (Lat/Lon) systems are often referred to as map
"projections". Technically, however, they are coordinate
systems rather than projections as defined above.
- The Universal Transverse Mercator (UTM)
coordinate system divides the globe into sixty zones,
each spanning six degrees of longitude. Each zone has its
own central meridian from which it spans 3 degrees west
and 3 degrees east. See the
Colorado State University UTM page for a complete explanation.
- Geographic (Latitude - Longitude)
is a coordinate system that treats the globe as a sphere
divided into 360 equal parts called degrees. Each degree
can be further subdivided into 60 minutes, each composed
of 60 seconds. The standard origin is where the Greenwich
Prime Meridian meets the Equator. All points north of the
Equator and east of the Prime Meridian are positive. The
origin divides the globe into four quadrants; northwest,
northeast, southwest and southeast. Each line of longitude
runs north and south and measures the number of degrees
east or west of the Prime Meridian. Values range from positive
180(eastern hemisphere) to negative
180 degrees (western hemisphere). Lines of latitude are
parallel to the Equator and run from east to west. They
measure the number of degrees north or south of Equator.
(A popular saying is "changes in latitude bring changes
in attitude.") Values range from +90 at the North Pole
to -90 degrees at the South Pole.
What is
a Datum?
The earth's surface is not perfectly
round, but shaped as an ellipsoid. Datums were developed
to accurately map topographic differences in the earth's surface
based on an ellipsoid. A datum is a set of parameters
defining a coordinate system, and a set of control points whose
geometric relationships are known, either through measurement
or calculation (Dewhurst 1990).
Most of the spatial data available from the USGS use one of
the following two horizontal datums used almost exclusively
in North America, the North American Datum of 1927 (NAD27) and
the North American Datum of 1983 (NAD83).
North
American Datum of 1927 (NAD27)
The North American Datum of 1927
uses the Clarke spheroid of 1866 to represent the shape of
the earth. The origin of this datum is a point on the Earth
referred to as Meades Ranch in Kansas. Many NAD27 control
points were calculated from observations taken in the 1800s.
These calculations were done manually and in sections over
many years. Therefore, errors varied from station to station.
There may be as much as 300 feet difference in positions based
on NAD27 compared to the more accurate NAD83.
North American Datum of 1983
(NAD83)
TerraServerUSA data are based
on NAD83. Many technological advances in surveying and geodesy
since the establishment of NAD27-electronic theodolites, GPS
satellites, Very Long Baseline Interferometry, and Doppler
systems-revealed weakness in the existing network of control
points. Differences became particularly noticeable when linking
existing control with newly established surveys. The establishment
of a new datum would allow for a single datum to cover North
America and surrounding areas, consistently.
The North American Datum of 1983 is
based upon both Earth and satellite observations, using the
GRS80 spheroid. The origin for this datum is the Earth's center
of mass. This affects the surface location of all latitude-longitude
values enough to cause locations of previous control points
in North America to shift, sometimes as much as 50 feet. A
ten-year multinational effort tied together a network of control
points for the United States, Canada, Mexico, Greenland, Central
America, and the Caribbean.
*The preceding definitions
are adapted from the Georgia
State GIS Data Clearinghouse website. To learn more about
projections, datums, or coordinate systems refer to: Peter
H. Dana, The
Geographer's Craft Project, Department of Geography, The
University of Colorado at Boulder.
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