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7/25/2008 11:02:16 AM

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  The National Agriculture Imagery Program (NAIP) is one of the largest orthoimagery programs in the country, boasting one of the fastest turnaround times from image acquisition to field usability. Administered by the U.S. Department of Agriculture's Aerial Photography Field Office in Salt Lake City, NAIP acquires imagery during growing seasons in the continental United States to meet Farm Service Agency (FSA) business requirements, and is used by numerous other federal, state and local governments, private businesses and individuals.

The challenges of providing timely delivery of a quality product are ever present. However, recent changes in imagery acquisition and image processing requirements have improved NAIP's services significantly.

Imagery Acquisition Advances
Notable imagery acquisition advances include a trend away from film acquisition toward digital acquisition, as well as the collection of image data in four bands. In addition, a pilot project created a seamline shapefile, outlining the areas of the imagery acquired by a digital camera for each flight line. Another pilot project investigated the use of Interferometric Synthetic Aperture Radar (IfSAR) imagery.

Film vs. Digital Imagery
In recent years, vendors have increased their use of digital cameras for NAIP imagery acquisition. The first digital project, Nebraska, was flown in 2003. This expanded to five states with digital acquisition in 2004, 11 in 2005 and 26 in 2006. Since 2003, acquisition has moved from 11 percent digital (89 percent film) to 53 percent digital. The estimate is for a move to 70 percent digital during the next two years.

In 2007, Arizona, Florida, Georgia, Indiana, Maryland, Mississippi, Tennessee and West Virginia received digitally acquired imagery. They all have a 1-meter ground-sample distance (GSD) except Indiana, which is 2-meter GSD. Illinois, Iowa, Louisiana, Maine, Missouri and Nebraska received imagery acquired with film cameras. Iowa and Nebraska received 2-meter GSD imagery, and the rest were 1-meter GSD. All 1-meter imagery is available to the public through APFO's Customer Service Section (sales)—e-mail: apfo.sales@slc.usda.gov, phone: 801-844-2922, fax: 801-956-3653.

Four-Band Imagery
A 2007 pilot project in Arizona delivered a four-band imagery product. Four-band imagery lets users select either natural color or color infrared in the application by changing the band assignments. The cost of adding the fourth band to the deliverable product isn't significantly more, as digital cameras acquire the infrared band at the same time as the bands for natural color.		  
   
 
  A problem with four-band imagery is that color balancing often is restricted to only three bands at a time—either natural color or color infrared would be balanced, but not both. Some vendors are working to address the problem by developing their own code for processing the imagery. Nine states are proposed for four-band imagery in 2008: Connecticut, Indiana, Kansas, Minnesota, Rhode Island, Tennessee, Texas, Vermont and Virginia.

Seamline Shapefiles
A 2007 pilot project in Arizona delivered ESRI shapefiles by county, delineating the parts of the imagery acquired by each pass of a Leica ADS40 digital camera (www.leica-geosystems.com). Many customers who have a need to know the acquisition date and time for different parts of the imagery have requested this information. The pilot will continue in 2008, testing the shapefile creation with different types of digital cameras. APFO plans to offer this capability in 2009.

IfSAR Pilot Project
Another pilot project used a dual-sided, dual-frequency IfSAR mapping system flown by Fugro EarthData (www.earthdata.com) in Yazoo County, Miss. The system was flown with the company's GeoSAR technology, which includes X and P bands.

The X band collects "scatter" off the first surfaces of vegetation, buildings, and bare earth, and the P band penetrates foliage and records substructures. The imagery will be used to develop and test an Automated Crop Cultivation Assessment Tool. In addition, the data could be useful in providing more accurate digital elevation models, especially in sparsely vegetated areas, and could be used for crop/acreage compliance, disaster relief or conservation practices.		  
   

 

  
  A New Approach to Image Processing
With each new growing season, APFO strives to improve NAIP products. In 2007, there were changes to the processes affecting tonal quality and positional accuracy.

Tonal Quality
For years, APFO provided contractors with image samples to match. Discussions of image quality were somewhat subjective, with no clear way to fairly evaluate the imagery as it was inspected. In 2007, FSA worked with ITT Space Systems (www.ssd.itt.com) to determine parameters for image quality. The company studied the color perceptions and preferences of FSA employees who use imagery and made image processing recommendations to meet their needs.

In early February, ITT delivered an assessment of best practices to be used in image creation, recommending ideal quantitative measurements for the image parameters and defining equations to be used in measurement. Four metrics implemented in 2007 were for histogram peak, color balance, contrast and overall clipping (see image samples).

Histogram Peak. The histogram is a starting point for digital image quality discussions. Digital imagery information is stored as a numeric value for each picture element (pixel). A histogram is a chart showing how many pixels contain each of the possible values within the image's range. These discrete values are known as "bins."

Because NAIP is 8-bit imagery (per band), there are 256 potential values for each pixel in an image. The three histograms of a color image display the distribution of pixel values for each band of the image—red, green and blue. The luminosity histograms shown at the bottom of the image samples at right are a composite for the three bands combined. To assure that an image is neither too dark or too light, the target for the histogram peak is within 15 percent of the center value.

Color Balance. ITT defined color balance as a "natural look of colors within an image without the dominance of any individual color." In a color-balanced image, the same metric value for each of the three bands would produce a neutral gray tone. The pixel values that actually produce a neutral gray in a given image should be close to each other, if not equal. The histograms of the different bands should have no more than ±10 pixel values between the "gray value," with a target of ±5 values difference between them.

Contrast. The contrast metric relates to the range of the maximum and minimum values of an ideal color range. Contrast would define the width, but not the shape, of the histogram curve. ITT determined that FSA users would prefer a minimum contrast of 120, with a target of 150. In the context of farm programs, contrast can assist with crop identification.

Overall Clipping. This metric refers to the number of pixels with values in the outer bins of the histogram. No more than 2 percent of pixels should be in the first or last five bins of the luminosity histogram, with a target of no more than 1 percent. Clipping can result from an overly aggressive contrast enhancement and creates a histogram curve wider than the 256 values allowed, with values above 255 or below 0. These values are assigned to the outer bins, resulting in a high number of pixels on the edge of the ranges of tonal possibility. Clipping on the left of the histogram heightens shadows, and clipping on the right intensifies highlights.		  
     
 

 

  
  Positional Accuracy
In the past, NAIP contractors were required to use the "best available" digital elevation model. In 2007, the contract was more specific, stating "the contractor shall use the most current/recent version of [U.S. Geological Survey] National Elevation Dataset (NED) for terrain-correcting the imagery."

A more important change, however, is the move from "relative" to "absolute" accuracy specifications. The major problem in making this change is finding accurate control points to be used by APFO inspectors validating the product's accuracy. There is no free, national, standardized, photo-identifiable control point database available for use in production or inspection. Obtaining control data requires additional time and resources. Moreover, additional image processing requires changes to APFO's inspection, database and contracting procedures.

APFO is continuing with the change to absolute accuracy specifications for the 2008 flying season. Imagery for seven states is scheduled to be processed and inspected using absolute ground control points: Indiana, Minnesota, New Hampshire, North Carolina, Texas, Virginia and Vermont. APFO's Service Center Support Section is working to create an in-house control database that will be used this year and in the future.

Imagery for the Nation
The National States Geographic Information Council hopes to incorporate NAIP into the proposed Imagery for the Nation initiative. This would include digital orthoimagery from different sources and at different resolutions. NAIP would be unique because it is flown under leaf-on conditions.

More information on Imagery for the Nation is available at www.nsgic.org. If this proposal becomes a reality, NAIP will fill a vital role in providing a comprehensive imagery source. 		 
   
     
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