Request For Proposals Aerial Acquisition Primer

Request For Proposals Aerial Acquisition Primer
March 27, 2008 Mike Tully

This is the fourth and final article in a series about writing Request for Proposals (RFPs) to ensure high quality geospatial services are acquired. In the previous issue, we discussed ground control requirements. In this edition, we will address details about aerial photography acquisition and complete our discussion on creating RFPs.

Flight Height, Photo Scale, & Positional Accuracy

The acquisition of aerial photography is one of the most expensive and important components of building a quality base map. Flight height, photo scale, and positional accuracy are important factors that affect quality photography and professional services derived from it.

Flight height influences photo scale and positional accuracy. The higher one flies, the more difficult it is to discern features on the ground. As altitude increases the maximum achievable positional accuracy decreases for a given airborne sensor, but the costs of the acquisition decline. Therefore, there is a tension between interpretability and accuracy on one hand and cost on the other.

Practical Flight Height Cost Concerns

The cost of acquisition increases as flight height decreases for a number of reasons. First, costs increase because more photography and more flight miles are required to cover the same area of the earth with photography. Second, more fuel is consumed at low altitudes than at high altitudes. Third, Federal air traffic control regulations may place costly restrictions on flying below certain altitudes.

Deriving The Correct Photo Scale

Photo scale is computed by dividing the above ground level (AGL) by the camera’s focal length. Common film aerial cameras have a focal length of 6″. Digital sensors have a variety of focal lengths commonly 60 mm – 120 mm.

Let us presume a mission is flown at 12,000′ AGL with a camera focal length of 6″ (0.5′). The photo scale calculation would be as follows:

The AGL and focal length are expressed in the same units and give the photo scale (1:24000) as a representative fraction (RF).

To convert this to a descriptive scale using some common unit of measure like “feet”, divide 24,000 by 12″ per foot to get 1″ = 2000′.

What About Pixel Size?

Perhaps the most misunderstood aspect of aerial photography is the relationship between pixel size, flight height, and positional accuracy. Many people mistakenly believe pixel size determines or influences the positional accuracy of digital imagery. This is mostly incorrect.

The resolution of the image may be changed to any desirable amount by changing the scanner resolution (or the resolution of the sensor unit in a digital camera). Resolution is not changed by adjusting the flight height.

However, the ground area covered by a single pixel of a given size is directly related to flight height.

For example, an image taken at 4,800′ AGL with an aerial film camera will have a photo scale of 1″ = 800′. If this frame is scanned at 1693 dpi (15 microns), then each pixel will cover 0.47′ on the ground. The table below lists the actual ground distance a single pixel covers, given some flight heights and scanning resolutions.

Ground size of each pixel given scanning resolution and photo scale
AGL (Feet) Photo Scale (Feet) Scan Resolution (Microns)
10 15 20 25
2000 333

0.13

0.20

0.26

0.33

2400 400

0.16

0.24

0.31

0.39

3520 587

0.23

0.35

0.46

0.58

4800 800

0.31

0.47

0.63

0.79

9600 1600

0.63

0.94

1.26

1.57

15840 2640

1.04

1.56

2.08

2.60

20000 3333

1.31

1.97

2.62

3.28

Many commonly confuse pixel size with accuracy. They think “that if only my photography had smaller pixels, then it will be more accurate.” Unfortunately, this is not correct. Pixel size is not directly related to positional accuracy.

Explained further, let us presume a photograph is taken from 15,840′ AGL and is scanned at 10 microns to produce 1.0′ pixels. A second photograph from the same camera is taken from 2,000′ AGL and is scanned at 76 microns to produce the same 1.0′ pixels. Positional accuracy of the lower photography with the very large and coarse pixels will be far more accurate than the high altitude photography with tiny pixels. This is because at low altitudes we can discern details on the ground with much more clarity. Manholes and fire hydrants could easily be seen and measured in the 2,000′ photos, but would not be visible in the image from 15,840′. As you can see, the size of each pixel has very little to do with accuracy.

Request Proper Camera Calibrations

Cameras must be calibrated. Once calibrated cameras are sometimes referred to as “metric” cameras. Calibration is the process by which the errors caused by distortions in its lens are objectively measured. All cameras require a lens through which light passes to expose a piece of film or a digital sensor. If a light ray cannot pass through a lens and predictably hit the film or sensor, the positional accuracy of the photography will suffer. When camera systems are calibrated these lens distortions are carefully measured and recorded. Modern mapping software then uses these calibrations to correct for error and ensure higher positional accuracies.

As camera systems are used, their properties change (i.e. lens changes shape). This is why metric cameras must be periodically recalibrated. If an uncalibrated camera is used on a project, it should be clearly noted. It may not be possible to achieve the desired positional accuracies if such a camera is used. The RFP should include a requirement to provide proof of calibration.

Aerial film cameras are calibrated every three years by the United States Geographic Survey (USGS) at their calibration facility in Virginia. This procedure was established decades ago to provide a standardized calibration procedure for the mapping industry.

A standardized calibration procedure has not yet been established for today’s digital cameras because they no longer share similar designs as film cameras have enjoyed for decades. Instead, digital cameras should be calibrated once each year by their manufacturer.

Example Flight Maps

Flight maps are created before an aerial acquisition takes place. These maps generally specify a variety of parameters including the flight height, distance between flight lines, distance between frame centers, frame overlap, mapping areas, and total area to be photographed. Your RFP should request a flight map is provided in a specific file format. To achieve this, you should provide the respondents both project and flight areas in digital format (CAD or GIS files) to eliminate confusion regarding what will be photographed and/or mapped.

Stereoimagery, Orthophotography & DEMs

Aerial photography can be acquired in two fundamentally different ways:

  1. Overlapping stereoimagery: 3D measurements are possible
  2. Non-stereoimagery: 3D measurements are not possible

Photography from either source can be used to create orthoimages. However, if non-stereoimagery is used for orthos, DEMs (digital elevation models) cannot be created. In this case, DEMs must be acquired from secondary sources. If stereoimagery is acquired your provider can create a new DEM. This new DEM will reflect the ground as it existed at the time of the photography and will be more accurate than acquired DEMs which may not show more recent changes to the earth’s surface features. In both cases the positional accuracy of the eventual orthophotography is dependent on the accuracy of the acquired DEM. If an accurate base map is the desired outcome, it is important stereoimagery is acquired so new or updated DEMs can be made.

It is critical for the RFPs to specify whether stereoimagery is required, and if a new DEM will be produced from the photography. Alternatively, the RFP may indicate an existing DEM will be used to orthorectify the photography.

Concluding Our RFP Series

We hope you enjoyed our four part series regarding how to improve and understand your RFPs. We have tried to help you learn how geospatial acquisition processes work and what aspects are important in your next RFP. If you missed any of the articles, please check them out at www.AerialServicesInc.com.

Questions Or Comments?

If you have questions about this article, feel free to contact Aerial Services, Inc.’s experts for advice with no obligation. We are here to help you.