Feature: Small and Large Coexist
Professional Surveyor Magazine - May 2009
Small- and medium-format digital aerial cameras have come on the scene in recent years, offering several advantages. But the large-format variety still do the bulk of the work.
By Bob Williams
During the past five years, small- and medium-format digital cameras have been introduced to the traditional photogrammetry marketplace. On occasion, they are used as a source to generate orthophoto imagery and precision mapping. In particular, sensor configurations used for oblique aerial photography have grown in popularity in the GIS and emergency response communities. Therefore, it only makes sense that companies like Sanborn educate potential users so they can make optimal decisions on purchasing equipment and get the most benefit from it.
Oblique photography has proven a great tool and a welcome addition to visualize properties, structures, and natural land formations from various angles and perspectives. The 3D-like imagery can be geo-referenced with side views for the structures, and ingress and egress for the property, entry into a building, and structure height and surface area all can be easily obtained from the oblique image perspectives. The visualization tool is a significant benefit to appraisers of real estate, emergency first responders, and site analysis for commercial investments as well as urban planners.
While oblique photography firms have used different sensor types and equipment configurations, they all have similar characteristics to accomplish the aerial photography mission. Normally, multiple small- or medium-format digital cameras are configured in a cluster or pod on the aircraft. One camera will face forward in the direction of flight, one backwards, and one each looking left and right along the flight line. These digital cameras are positioned at 45-degree angles to obtain the vertical oblique perspectives. The images are normally captured as a low oblique (no horizon), and they are typically high resolution (3degrees or 6degrees) due to the low altitude of the flight mission. In addition to the oblique sensor configuration, a nadir, or downward-pointing, camera is normally in the vertical position.
It is important to understand the impact of using the down or nadir digital camera for generation of precision orthophotos. Advantages and disadvantages exist for using a small- or medium-format digital camera to generate orthophoto map products that can be certified to an acceptable industry standard, including NMAS, ASPRS, NSSDA and others.
Small and Medium Advantages
The advantages of using a small-format camera can be characterized by identifying unique components by which oblique data is collected.
Single Mission: Vertical photography can be obtained during the same mission as oblique imagery, reducing mobilization and possibly lowering project costs, depending on the design and requirements of the project. This may be the single greatest advantage.
High Resolution: Since oblique imagery is normally taken at low altitudes (below 2,500 feet) and with a digital camera, the image quality is excellent. Typically, pixels obtained from the vertical camera will have a ground resolution of three to six inches and are collected as a true color image (RGB).
Meets Mapping Standards: If the raw imagery is processed using proven photogrammetric techniques, including survey control and an accurate digital elevation model (DEM), the orthophoto mapping can meet published accuracy standards.
Small and Medium Disadvantages
But using small- and medium-format cameras has disadvantages as well. The cumulative impact related to the size of the camera footprint results in technical difficulties and increased time to produce an orthorectified product. Key issues relate to:
Image Footprint: The footprint of the small-format camera pixel array combined with the low altitude of the aerial mission will increase the number of flight lines and digital frames by a factor of four to eight, depending on the resolution requirements. This results in higher flight and downstream production costs because of the quantity of exposures that must be rectified, mosaiced, and color-balanced.
Camera Calibration: Currently, the small- or medium-format cameras in use today for collection of oblique photography missions are not calibrated or certified by a national agency such as the USGS.
Focal Length: The focal length on the vertical (down) camera ranges from 1.5 to 2.25 inches, creating a super-wide angle. The sharp angle increases the radial displacement for above-ground structures. This causes excessive building lean in urban areas. While this may be partially offset by gathering digital frame exposures with a heavy forward overlap along the flight line, the super-wide angle increases the parallax between exposures when using the imagery in a photogrammetric production environment. Parallax greatly affects the ability to make precision measurements from the stereo imagery.
Base-to-Height Ratio: Additionally, the short focal length of the small-format digital camera produces a poor base-to-height ratio, further derogating the ability to make precise measurements.
Precision Measurements: Due to focal length, camera calibration, and base-to-height ratio, these sensors are not suitable for the generation of a DEM or updating an existing one. The DEM is a critical component to the generation of accurate orthophoto imagery. Additionally, planimetric feature extraction is not reliable or as accurate.
Mosaicing: As a result of the volume of exposures, the mosaicing between frames is excessive and causes problems with distorted buildings and above-ground features. In many cases, large buildings can't be gathered from a single exposure and must be mosaiced from multiple exposures. Consequently, the building may be grossly distorted in appearance.
Color Balancing: Color balancing between exposures and within an orthophoto tile requires multiple reiterations to achieve acceptable radiometric balancing.
Automated Aerial Triangulation: With the radial distortion of a less-expensive camera lens and the super-wide angle, automatic pixel matching for aerial triangulation and camera orientation often become a manual process, increasing the cost of the finished orthophoto product.
3D Buildings: While the multiple perspectives give the appearance of 3D, the imagery is still 2D in nature. True 3D buildings can be extracted from the imagery using photogrammetric techniques, but the structures will yield the same inherit decreased accuracy for reasons stated previously.
Vegetation: Because oblique imagery is often taken during leaf-on conditions, the imagery is not ideal for viewing ground features. Leaf-free imagery for orthophoto imagery has been a fundamental requirement and remains important for most mapping applications.
Flight Procedures: The flight requirements for the oblique imagery often require the aircraft to mirror the terrain changes versus flying at a constant height. This technique creates varying side lap and forward overlap in the vertical imagery and complicates the photogrammetric process for generating stereo imagery. Constant flight with small variance in flying height has been mandatory and a common practice that should not be ignored.
When firms offering small-format oblique photography use the term "orthophoto," the end-user should proceed with caution and ask technical questions to ensure they are receiving the product accuracies and quality they expect. All orthophotos and pixels are not created equal. As the perfect mousetrap does not exist for simultaneous collection of small- to medium-format obliques and large-format precision vertical imagery, it's still best to contract for different missions to achieve similar but different objectives. The sophisticated GIS community is not ready to settle for less quality or accuracy for the orthophoto imagery at the expense of gathering oblique imagery. Many clients continue to require the highest quality and accuracy at a reasonable cost.
The oblique imagery is a valuable addition to the GIS database, but similar to lidar data, the oblique mission still requires a separate flight to best achieve a different objective. The vertical aerial photography mission should create the highest quality image product that meets the precision and accuracy of a photogrammetrically prepared base map. This goal is best accomplished by using a large-format, precision digital mapping camera.
All large-format digital mapping cameras on the market today have a longer focal length, a manufacturing process for sensor assembly certified by the USGS, and a larger image footprint. These combined factors enable the typical photogrammetric engineering process to be maintained, including precise horizontal and vertical measurements to produce topographic, planimetric, and true 3D features, including buildings.
Questions to Ask the Small-format Photography Vendor:
- Is the down or vertical camera calibrated or certified by the USGS?
- What is the camera focal length and at what height will the mission be flown? How much deviation in altitude will occur in the mission? What technical specifications will be used to control the aerial mission?
- How will a new DEM be created or updated using a medium-format digital camera?
- What is the absolute horizontal and vertical accuracy of the finished image product?
- What field-testing or other QA is planned to achieve the accuracy goal?
- Can planimetric and topographic features be stereo digitized from the imagery to update the GIS database?
- Is there a Certified Photogrammetrist or Registered Land Surveyor supervising the production of the mapping products?
We see that small- and medium-format digital cameras are here to stay and will continue to grow in use because of the advantages they offer. But we can also see that large-format cameras are still required for many applications, and in many cases, the two camera types should work together and complement each other in the aerial surveyor's arsenal.
Bob Williams is senior vice president and general manager for Sanborn Imagery Services Central in St. Louis, Missouri. He is a certified photogrammetrist, a professional photogrammetrist in Oregon, and a professional surveyor and mapper in Florida.
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