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FAQs on Aerial Photography and Related Subjects Oregon/Washington BLM



FAQs on Aerial Photography and Related Subjects

Aerial Photography

What different services does Mapping Sciences offer?

The Mapping Sciences Section's Aerial Photography Group establishes, monitors, and manages contracts for aerial photography. The products we offer at this time are: 9"x 9" aerial photos in natural color or color infrared, with stereo coverage; diapositives; photographic enlargements; oblique aerial photographs; digital ArcView indexes of your aerial photo project (since 1999, some available for older projects); or hardcopy project indexes. Aerial photos may be laminated, mosaicked for display, or mosaicked in black and white to be used as an index. Traditional aerial photos may also be scanned, rectified, and put into a GIS. Mapping Sciences has purchased a 12"x18" scanner and ER Mapper software for rectifying and digitally mosaiking. At the present time, Mapping Sciences contracts the scanning, rectifying, and mosaicking using assistance from an on-site contractor.

In addition, the Aerial Photography Group can help in locating historical aerial photographic images taken by the BLM or other government agencies.

What information do you need from me if I want to have an area flown to acquire new aerial photos?

Scale, type of film (color or color infrared), project boundary, preferred time of year, and your intended end use. The most important of these is end use. If you need to identify objects on the ground, what is the diameter of the smallest object that you need to see discretely? In the Northwest, most photography is flown in the summer. However, if you have a specific need for photography in another season, it can be done, with adequate advance planning. If you want to study a single species, be sure to pick the time of year that it may be most easily distinguished. If you can provide the Mapping Sciences staff with your specific end use(s) and a project boundary, we can determine the product and scale that will best suit your needs.

I've seen scale expressed in two ways. Do you have a cross-reference?

Scale can be expressed as a representative fraction such as 1:1,000. This means 1 unit on the photo is equivalent to 1000 units on the ground. This is true whether the units are miles or millimeters. People who deal with large scales and need to measure them are more familiar with English equivalency scales such as 1" = 50 feet; that is, 1 inch on the photo is equivalent to 50 feet on the ground.

Representative Fraction English Equivalency
1:2,400 1" = 200'
1:4,800 1" = 400'
1:6,000 1" = 500'
1:12,000 1" = 1,000'
1:15,840 1" = 1,320' or 4" = 1 mile
1:20,000 1" = 1,667'
1:24,000 1" = 2,000'
1:40,000 (NAPP) 1" = 3,333'

How much will my project cost?

There are many variables that affect cost such as in what part of the state your project is located; whether the terrain is flat or mountainous, timbered or dry; the shape and size of your project boundary; what altitude the pilot will need to fly; what scale you require; what type of film is used; what products you need and how many of each; and whether you need your photos laminated. The Mapping Sciences staff have contracts in place with prices based on the variables noted above. We can typically provide you a cost within 1 to 5 days.

How can I get more bang for my buck?

The smaller the scale you request, the higher the plane will fly and the more area you will see in one photo. Thus, fewer photos are needed. It is always prudent to select the smallest scale for your use.

Natural color photography is significantly less expensive than color infrared. Color infrared processed to a negative is less expensive than color infrared processed to a positive.

How do I determine the best scale for my needs?

If the product is for a number of different uses, 1:12,000 is an ideal scale. If you want to use it solely for identifying specific features on the ground, determine the diameter of the smallest feature you need to identify. For other uses, the following chart can be used as a very general guideline:

End Use for Photography Color IR Natural Color
Identify conifers with crown diameter 2 feet or larger, for plantation management 1:7,500 1:6,000
Identify individual trees, bushes, or other features for project boundary locations (crowns 3 feet or more in diameter, features 3 feet or more in size) 1:8,000 1:7,000
Identify existing snags, 20 inches or more in diameter, 20 feet or more in height 1:8,000 1:8,000
Identify slope breaks for stream management zones 1:24,000 1:11,000
Identify tree species (for groups of trees) based on crown forms, texture, and color (crown diameter 20 feet or larger) 1:18,000 1:11,000
Determine wet areas 1/10 acre or larger in size by color, texture, species composition of surrounding vegetation 1:16,000 1:12,000
Locate existing trails, ditches, and other linear features 3 feet or more in width, broken to open cover 1:15,000 1:13,000
Detect erosion, large gullies, etc. (8 feet or more in width) 1:15,000 1:14,000
Identify rock outcroppings or obstructions 30 feet or more in size 1:15,000 1:15,000
Determine crown density (areas 1 acre or larger) 1:16,000 1:15,000
Identify potential rock source location (areas 1 acre or larger) 1:27,000 1:25,000
Locate change in vegetation cover in areas 5 acres or larger for fire rehabilitation planning 1:30,000 1:25,000
Locate existing structures - buildings, water towers, etc. (20 feet or greater in width or diameter) 1:30,000 1:24,000

Can you explain the different types of photo project indexes available?

Our standard photo project index is a paper copy (made from a mylar original) identifying the flight lines and photo centers on a map base. All new projects of more than a dozen photos are also available in an ArcView digital file. We are expecting eventually to phase out the mylar indexes in favor of the digital ones. We will still be able to print paper indexes from the digital files.

We can also order a black and white photo mosaic in which the actual photos are reduced to approximately postage stamp size and laid out atop one another with the titling showing. This is not to be confused with a display photo mosaic which is not an index.

What kind of turnaround time can I expect?

Because most aerial photography is taken during the summer, the workload is concentrated for both contract firms and the Mapping Sciences staff. Expect at least 4 weeks for a small natural color project. Rush jobs for law enforcement, court cases, and the like can be accomplished in less time.

Can I use aerial photography to make orthophotos?

Yes and no. A true orthophoto is created by tying each photograph diapositive to known geographic coordinates on the ground and eliminating displacement caused by uneven terrain and distortion caused by the airplane's position (yaw, pitch, and roll). It is very time consuming but will accurately match rectified maps such as the USGS topographic series. The digital ortho quads (DOQs), available on disk, were created this way from 1:40,000 scale black and white NAPP (National Aerial Photography Program) taken in 1994 (Oregon) and 1996 (Washington). These DOQs will need to be used until further notification.

An ortho-like product can be produced from aerial photos using georeferencing software. This product takes less time and should be "good enough" to use for tasks that don't require exact positioning such as vegetation analysis. Georeferencing ties the photo to geographic coordinates but does not correct for terrain displacement or the airplane's position. These products can also be mosaicked to form a layer or theme in a GIS. They may not match other layers/themes that are rectified.

How can I scan air photos in my own district?

To do it right, start with diapositives. These are film positives of the air photos, visually similar to what 35mm camera users call "slides." They are on a stable base material, about twice as thick as the original film, so are less subject to the vagaries of temperature and humidity than photos printed on paper. Use a backlit scanner to get the best color, resolution, and depiction of fiducial marks. With some cameras, the fiducial marks are only very tiny dots which may not be easily discernible on prints but will be discernible on diapositives. Backlighting will emphasize these dots.

Some users have elected to scan paper prints to reduce the cost of procuring diapositives and with the realization that the resulting orthophotos will be somewhat inaccurate. One difficulty in this method is that the fiducial marks may be so small and faint that the scanner does not detect them. Lance Finnegan, in the Coos Bay District, found a time consuming but elegant solution to this problem. He lightly punched each fiducial center point with a fine needle. The resulting scan showed the needle prick.

The scanner can be set to any resolution that suits your needs. The higher resolution yields better detail but larger file sizes. Many scanners have a maximum scanning resolution for actual reproduction; higher resolutions will create a larger number of pixels but the pixels are interpolated. Do not exceed your scanner's scanning resolution for actual reproduction. Past projects have been scanned at 300 and 600 dpi (dots [pixels] per inch). Sometimes scanning resolution is noted in microns or lines per inch. An equivalency table is shown below:

DPI (Dots Per Inch) Micron Scan
100 254.0
200 127.0
300 84.7
400 63.5
500 50.8
600 42.3
700 36.3
800 31.8
900 28.2
1000 25.4



How do I read a BLM air photo index?

The traditional BLM system reads as RR-FF-EE where RR represents the roll number, FF represents the flight line, and EE represents the exposure number. In 1995 and 1996, however, the system was changed to match the system used by the US Forest Service. It reads RR-EE.

Individual BLM photo exposures are numbered south to north with the flight line numbers starting in the southwest corner. In a few other cases, they are numbered from east to west with the flight line numbers starting in the southeast corner.

How do I read north on an air photo?

Titling is done at the north edge for south to north flights and at the west edge for east to west flights. Riparian photography is titled on the upstream edge with no cardinal direction indicator.

How much area does an air photo cover on the ground?

On flat terrain, the following ground measurements apply to a 9"x 9" air photo:

Common Photo Scales Ground Dimension on One Side, in Feet Ground Dimension on One Side, in Miles Ground Area, in Acres Ground Area, in Square Miles
1:2,400 1,800 0.34 74 0.11
1:4,800 3,600 0.68 297 0.46
1:6,000 4,500 0.85 464 0.72
1:12,000 9,000 1.7 1,859 2.90
1:15,840 11,880 2.25 3,240 5.06
1:20,000 15,000 2.84 5,165 8.07
1:24,000 18,000 3.4 7,438 11.62

What are "control panels" and when are they needed?

Control panels, sometimes called "ground targets" are markers on the ground that pinpoint known geographic positions such as survey monuments. The markers can be seen in aerial photos and are used to tie the photos to established x and y or x, y, and z coordinates. Control panels are needed to rectify the photographs, to put them into a GIS, and to make maps for accurately measuring. If an exact photo scale is important for a particular area, set a pair of control panels there, spaced at a known distance. Control panels set for this purpose do not need to be surveyed.

How do I make the control panels and set them up?

Control panels are typically 2, 3, or 4 legged and are centered on the monument or otherwise surveyed position as in the following examples:

Panel Dimension Template

The legs are made of heavy paper like waxed butcher paper (or preferably poly-based plastic made for the purpose) material that is secured to the ground sufficiently to keep them from blowing about in a wind. The material must contrast with the color of the ground so that white material is used for dark colored ground and dark material for light colored ground. White is most commonly used. Paper legs are usually held down with rocks. Plastic legs are secured with 20-penny (6-inch) nails driven into cardboard milk bottle caps (or 2 inch squares of stiff cardboard). Drive the nails into the cardboard "washers" before hammering them into the ground. Judgment should be used as to the spacing of the nails or rocks, depending on weather conditions or animals in the area. At very large photo scales (1:2,000 - 1:3,000), large semi-truck tires may be used in place of "legs." Be careful to center the tires on the monument and to paint the tires white (unless the ground is very light). Paint can also be used to mark a site on a road. The dimensions of the legs are dependent upon the scale of the photography. Use the following chart as a guide:

Aerial Photo Target Graph

To set them up, you will need at least 5 panel sites per project (more if project is large). The sites need to be near the corners of the project and in the center. The markers must be placed where they will be visible from the air and away from any possible shadows that would occur between 10:00am and 2:00pm.

Keep detailed field notes of the locations of the markers with drawings that may be used if it becomes necessary to relocate the spot. Surveyors from the OSO Cadastral Branch may be called upon to identify the geographic positions of the sites (coordinate through Ed Zigoy at 503-952-6142). If there is a need to rush the aerial photography, the sites may be surveyed after the photos are taken; however, the fragility of the markers necessitates the surveying be done with as little delay as possible. Once the photography has been completed, the panels or tires should be removed.

Can I search for a BLM air photo on the Intranet?

The Mapping Sciences Section is mapping aerial photo project boundaries within the states of Oregon and Washington on our website. We have identified selected major projects, covering the last 50 years. We will be adding new projects and selected older projects as time allows. We are in the process of setting up all of the cyclical projects from Medford 1996 through the present Eugene 2000 in an ARCIMS format that allows you to view specific layers and to zoom in to see individual photo centers. To date, none of the other BLM state offices have an index to aerial photography on the Internet. The film for virtually all of the projects shown on the Oregon/Washington website is held in Denver. Call Connie Slusser (303-236-7991) or Larry Cunningham (303-236-6382) for questions regarding BLM photography in other states.

How do I order aerial photos of past projects?

All Oregon project film older than one year is in Denver. BLM employees (and contributors to specific western Oregon cyclical projects) pay $10 for up to the first 3 natural color contact prints. All prints after that are $1.50 each. Enlargements are priced as noted below. The Denver photo lab does not trim the prints so there is a white margin all around. Trimming and laminating are extra and can be done in Denver, in Portland, or at a local contract laminator of your choice. Turnaround time varies considerably. To order, provide a list with the project code, roll, flight, and exposures you desire, preferably in order by roll number, to Connie Slusser at 303-236-7991. Please note that the pricing for non-BLM employees differs from the prices below.

Product Denver BLM Price for BLM Customers
9"x9" color diapositives (3 or less) $30.00
additional diapositives $ 6.00
12"x12" color paper print $15.00
18"x18" color paper print $20.00
27"x27" color paper print $30.00
9"x9" b/w paper print (3 or less) $10.00
additional b/w prints $ 2.00
9"x9" b/w film positive $20.00
9"x9" b/w film negative $20.00
12"x12" b/w paper print $12.00
18"x18" b/w paper print $15.00
27"x27" b/w paper print $20.00

Prints of past projects listed on the Mapping Sciences website (other than the cyclical photography) can be obtained by calling the Mapping Sciences staff. Pricing varies depending on the contract printer we use but will be more than the Denver lab prices. Turnaround time is much quicker. Call Ed Zigoy at 503-952-6142 or Susan Nelson at 503-952-6139. Prints ordered through our office do not need to be trimmed. Laminating may be ordered separately.

Why does the scale seem to change along the flight line in my project?

The distance between the camera and the ground determines the scale. The contract requires the contractor to fly a certain number of feet above the average ground elevation. For this reason, the scale on the hilltops will always be larger than the scale in the valleys. This situation is common in western Oregon where the terrain is rolling with mountains and valleys close to one another.

In a more simple example such as Steens Mountain, a pilot flying perpendicular to the mountain will fly at one elevation up to the edge of the fault block to maintain the proper scale over the mountain. The pilot would fly a series of flight lines at the "mountain" altitude. Then the pilot will drop down and fly a series of flight lines at a lower altitude to maintain the proper scale over the Alvord desert. Both flight line segments would overlap. The point on the flight line where the altitude changes is called a datum break.

I need more information about BLM air photo projects. How can I get metadata?

For most projects there are two metadata files. One describes the photo images and the other describes the digital index.