About Orthophotography

 

What is Digital Orthophotography?

Digital orthophotography provides all of the visual content of a photograph while being as accurate as a map for measurements. These qualities allow for accurate:

  • distance measurements
  • area calculations
  • determination of feature shape
  • direction calculations
  • determination of coordinates at a given location

Digital orthophotography is an essential basemap layer for a geographic information system (GIS). The 2005 orthophotography will be seamless within jurisdictions and across the state. Free software to view the data is available.

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What is Pixel Resolution?

Digital orthophotography is a computerized image made up of pixels (these are similar to the dots on a television screen that make up the whole picture). Each pixel in an orthophoto represents a true distance on the ground. Thus, 1-meter pixel resolution means each little pixel in the image covers 1 meter on the ground. As you zoom in close to an image, you start to see the pixels (like getting really close to the TV screen). The higher the resolution, the more ground detail you can see as you zoom in closer on an image.

The 2005 orthophotography will be produced at different pixel resolutions based on county population and upgrade options (1-meter, 1-foot, and 6-inch). The following diagram demonstrates the differences in images at the three resolutions as you zoom in closer on the map.

1-Meter: Good/Medium Resolution

Uses include emergency management, planning, data verification and updates, large-scale analysis, change detection
Smaller file sizes

1 meter sample resolution

1-Foot: High Resolution

Uses include emergency management, and utility mapping, verification and updates
Larger file sizes

1 foot sample resolution

6-Inch: Very High Resolution

Uses include detailed project-level infrastructure mapping
Largest file sizes

6 inch sample resolution

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What Is Accuracy? Why Is It Important?

In plane English, accuracy describes how accurately a point on the map is located compared to the actual location of that point on the ground. Technically, accuracy of orthophotography is described in terms of its root mean square error (RMSE) (the higher the error, the lower the accuracy). This is very important for properly locating features on the ground, and for overlaying or creating other accurate data in a GIS system.

The 2005 orthophotography will have the following accuracy specifications that meet or exceed well accepted standards for accuracy:

Pixel Resolution (Ground Sample Distance)

1-meter pixel
1-foot pixel
6-inch pixel

NSSDA Horizontal RMSE Accuracy (95% of points)

 33.3 feet or better
5 feet or better
2.5 feet or better

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What Is A Digital Elevation Model (DEM)? How Is It Used?

A DEM is a model of the earth’s surface which is used to remove distortions in the aerial photography caused by changes in land elevation (valleys and ridges). The DEM may include treetops, rooftops, and tops of towers, telephone poles, and other natural or manmade features; or it may include the ground surface if there is no vegetative ground cover. The DEM allows photography taken of a 3-dimensional surface to be rectified to an accurate 2-dimensional photo-map. The DEM used for ortho-rectification of the 2005 orthophotography will be delivered as part of the project.

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What’s the Difference Between Black-and-White, Color, and Color Infrared Orthophotography?

Orthophotography can be produced with different imagery types. The 2005 orthophotography will be produced in true color statewide.

True color imagery shows the ground conditions in colors as seen by the naked eye. Color imagery is valuable because people can more quickly and easily interpret what they are seeing on a color image than black-and-white or infrared. Additionally, features appear more defined allowing people to see more features even at lower resolutions. Color works especially well in manmade environments. Color imagery produces larger file sizes compared to black and white. Fortunately, advances in file compression techniques and computer storage make color imagery within the grasp of common users.

Black-and-white imagery (sometimes referred to as grey-scale or panchromatic) is the traditional film type people are most used to seeing for orthophotography. The 1-meter statewide orthophotography produced in 1998 were black-and-white (1998 black and white, and 2003 color, 1-meter orthophotos are available for download free of charge from www.indiana.edu/~gisdata/). Black-and-white imagery is generally less expensive and requires about 1/3 the file storage capacity of true color imagery.

Color infrared imagery is sensitive to green, red, and near-infrared portions of the light spectrum. As such, the colors on the imagery appear un-natural, but they show information about vegetation health, water and water content, impervious surfaces for water runoff, and other information not visible to the eye. Grey-scale images can be produced from color infrared imagery. Color infrared is not a base product of the 2005 Orthophotography program, but might be included as an optional product (to be determined during project contract negotiations).

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How Is Orthophotography Used?

Orthophotography is part of the basemap in a geographic information system (GIS). It is used like a photograph as a visual reference, and since it has the qualities of a map it is used to generate other important mapping data. Mapping can be used for emergency response planning and modeling, response, as well as recovery. Law enforcement can use it for mapping trends, thereby improving performance, and public health agencies can use it for syndromic surveillance. And because the basemap is shared with other local, state and federal agencies, the GIS systems can be used for other applications as well, such as property management, Census, tax assessment, flood mapping, planning, and economic development. By having different agencies reference the same basemap, data developed independently can fit together.

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How Do I Get More Information?

Want more information? Contact us. We’ll be happy to discuss the project details with you.