How do I use the photo index?
A photo index consists of individual air photos fitted and layered together
to form a photo mosaic. Each individual photo was stapled to a board and
overlaid to allow the flight line and photo number to be seen. To find
the area of a county you are seeking, you will need to select the portion
of the county you would like to see by clicking on the small map of the
county. Secondly, you will need to orient yourself through landmarks and
forms to choose your area of research. Rolling over the index with your
mouse, you will be able to click on the the section to bring you to the
corresponding photo. If your cursor does not turn into a hand symbol (specifying
a link), then that individual photograph is unavailable in this database
at this time.
What if I can't locate an area on the index? Find a reliable and detailed county map, or a topographic map of the
quadrangle containing the area of your research site. Locate your site
and compare features of the map, such as road, streams and towns to the
same features on the air photo. You should be able to find similar features
on the index.
How do I read an air photo? You will need to identify road or railroad patterns first, then proceed
to drainage vegetation features. Once you become initially oriented, then
search for cultural features such as buildings and crops.Also,
try comparing the patterns on the photo to a map of the county, on which
you have already located your research site. You will need to adjust for
the difference in scale. For instance, a white dot on a photo may be a
house, a car, a sheep or a gravestone.
What are the numbers on the top of the photos?
The printed numbers at the top of a photo can vary from project to project.
The standard arrangement on the ASCS photography is to give the date the
photo was taken on the top left side (in Month, Day and Year) and a code,
consisting of a county identifier, flight line number and individual photo
identifier. Each county in the U.S. was given either a two or three-letter
code as identification, such as: CDR. With the introduction of the FIPS
code in the 1970's the letter code was changed to a five number code,
such as 13001. For example, if a code at the top of a photo is [2-23-40
CDR AA2 42] this indicates the photo was taken February 23, 1940,
the county is Appling County, Georgia, the flight line number is AA2 and
the photo number is 42.
Which direction is North on the photos? Most of the photos are oriented with North at the top of the photo.
On the ASCS photography, the top of a photo is the edge with the printed
photo identification codes. The indexes have North arrows and one should
always check the alignment of the photos and the North arrow to get the
Northing. If you need precise North measurements, you should compare the
photo image to a USGS topographic map.
At what scale were the air photos taken? Most of the pre-1970's photos are 1:20,000 scale while most of the
1980's and recent are 1:40,000 scale. The page that lists the indexes
for a year for a county also give the scale of the photographs. Also,
the master
list of air photos held by the Map Collection gives the scale of the
photos for each project.
How can I measure the scale of the air photo
I've downloaded? The scale of an aerial photograph can be calculated from the relationship
between a photo measurement and a measured ground distance, and is usually
given as a Representative Fraction (RF), such as 1/20,000. The RF equals
photographic distance between two points over ground distance between
the same points. For example, the distance between two road intersections
might be measured on a vertical aerial photograph as 0.6 inches. The corresponding
ground distance is measured as 15,840 inches. The scale (RF) would be
computed as:
RF = 0.6/15,840
RF= 1/26,400 or 1:26,400
In measuring RF one must use the same units of measurement in both numerator
and denominator. Where expensive ground surveys are impossible, ground
distance can be computed from a reliable map, such as an USGS 1:24,000
topographic quadrangle sheet.
Changes in the height of the terrain covered by the photo and any slight
tilt in the aerial camera can cause slight changes in scale across the
photo. Therefore it is best to do three different scale measurements and
average the three to get an overall scale for the photo. Also the points
selected for measurement should be diametrically opposed so the line connecting
them passes near the center of the photograph. Points that are approximately
equidistant from the center will help minimize the effect of photographic
tilt.
To get ground distance from a USGS topographic map measure the distance
between the two points using an ruler that measures in at least 1/10th
an inch. Measure the distance on the map and express that distance in
inches down to the one hundredth if possible. The scale of the map will
be given as a RF. If the map is an USGS 1:24,000 scale topographic quadrangle,
the RF will be 1:24,000, or 1/24,000.
For example, if the distance between two points on a 1:24,000 scale map
is 3.65 inches, then the distance on the ground is calculated as:
3.65/ ground distance(gd) = 1/24,000
3.65 x 24,000 = gd x 1
gd = 87,600 inches
Table 1: Scale Conversions for Vertical Photographs and Maps
Scale (RF)
Feet per inch
Inches per mile
Acres per square inch
Calculations=
RFD/12
63,360/RDF
(ft/in.)(ft/in.)/43,560
1:1,000
83.33
63.36
0.16
1:2,000
166.67
31.58
0.64
1:3,000
250.00
21.12
1.43
1:4,000
333.33
15.84
2.55
1:5,000
416.67
12.67
3.99
1:10,000
833.33
6.34
15.94
1:15,000
1,250.00
4.22
35.87
1:20,000
1,666.67
3.17
63.77
1:24,000
2,000.00
2.64
91.83
1:25,000
2,083.33
2.54
99.64
1:50,000
4,166.67
1.27
398.56
1:75,000
6,250.00
0.84
896.75
1:100,000
8,333.33
0.63
1,594.22
RFD = Representative Fraction Denominator
Note: There are 63,360 inches in the land mile used in the U.S.
Can I measure distance on the photos?
You can best measure distance from the photo by comparing the photo to
a reliable base map, identifying the points you see on the photo with
the corresponding points on the map. Measure the distance on the map.
Then use the scale on the map to compute the distance. Using the scale
bars on the map to get the distance is less accurate. You can use the
photo directly to measure distance by printing the photo and using the
print. Find the scale of the photo print and then use the following:
Distance in feet per inch = RFD/12.
RFD is the Representative Fraction Denominator, or the denominator in
the scale (For example, if the scale is 1/24,000 (often given in the form
1:24,000) the RFD is 24,000).
Distance example: If you measure a distance of 3.65 inches on a photo
print that has a scale of 1:24,000, then:
gd (distance on the ground, in feet per inch) = 3.65 X 24,000/12
gd = 3.65 X 2,000
gd = 7,300 feet
gd = 87,600 inches
Can I get latitude and longitude or GPS coordinates
from the photos?
You can not currently get coordinates from the online aerial photographs.
You can compare the photograph to an USGS topographic quadrangle map,
or another reliable map, find your point on the map, and then measure
the coordinates from the map. If you do not have a reliable paper map
available, you can get a fairly close reading by using online topographic
map databases, such as Topozone.
Why are only some aerial photographs online? The Georgia Aerial Photograph Collection currently contains only a
part of the collection held by the Map Collection at the University of
Georgia Libraries. A fairly comprehensive
list of the aerial photographs and satellite imagery held by the Map
Collection can be found on their web site. Also, some photographs from
a set were unavailable at the time they were digitized.
Do you have any photos earlier than the ones
you list?
A list of the Map Collection’s aerial photography holdings can be
found at the Map
Collection’s web site. The list contains about 99% of the Map
Collection’s aerial photographic holdings. The collection holds some
photography for highway sites and an extremely limited number of individual
photographs of towns and sites. The Map Collection is always
interested in obtaining any early photography of Georgia.
How do I get the latest photos of an area?
The latest photos of an area are probably not held in the Map Collection’s
holdings. The United States Geological Survey has a program, called National
Aerial Photography Program (NAPP), that provides new air photo coverage
of the United States on a regular schedule. Some of this photography can
be seen online at the Terraserver
site. The USGS records most federal photography in the Aerial Photography
Summary Record System (APSRS) and can be searched using Earth
Explorer. Regional, state, county, and city government agencies, mainly
Planning Commissions, GIS Database sites
and private companies can also provide coverage, especially of major cities,
but usually do not provide free online access to their holdings.
What is a MrSid image?
MrSid is developed and maintained by Lizardtech.
This file format allows the viewer to zoom in or out on each image to
see finer detail. Also, this format allows the user to print a zoomed
and/or cropped portion of the image to any available paper size. Most
browsers should be able to view the MrSid images on this database without
a plug-in.
Can I print or download an online aerial photo?
You can print or download the image to your computer without copyright
restriction, unless specific restrictions are applied on the page. To
download, right mouse click over the image and select "Download Image
to Disk" or "Save Image As" and save to your computer's
hard drive. To print, you will need to use the browsers print feature
under the 'File' menu. Keep in mind that your print will be identical
to the image you see on your screen, so if you have zoomed in on your
image, the resulting print will be zoomed in as well.
How can I get copies of aerial photos?
The Map Collection does NOT have facilities to make copies of the photos
it holds. You must go to the federal or state agencies that originally
produced the aerial photographs and purchase copies from those agencies.
