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**Irregularities in Replacement Imagery**

**INFORMATION SHEET**

**June 2008**

-   **What is Replacement Imagery?**

> Replacement imagery, also known as image base replacement in the USDA
> Farm Service Agency, is imagery that takes the place of another
> imagery dataset covering the same location on the earth's surface.
> Replacement of imagery may be due to acquisition of imagery that is
> newer, more spatially accurate or containing more information. It
> could also be due to a change in FSA program policy.

-   **Why are there irregularities in replacement imagery from one year
    to the next?**

> Virtually always, when new imagery is acquired, it does not match up
> spatially in every location with past imagery. It can be infuriating
> to digitize based on an image, and then be obligated to re-digitize
> the same area to match newer imagery.
>
> This is the nature of imagery acquisition. *No* imagery will look
> alike or be in the same exact location on the face of the earth
> uniformly, as compared to any other imagery dataset of the same
> location on earth.
>
> The only way two images could be exactly the same is if imagery was
> shot from the same exact location above the earth's surface at the
> same time of the day and same day of the year, from the same camera
> calibrated in the same manner, during the same atmospheric conditions,
> using the same orthorectification process and the same Aerial
> Triangulation (AT) solution, the same ground control, and the same
> Digital Elevation Model (DEM). Even then the two images would likely
> not look the same. Many of these variables are discussed later on.
>
> As with any photography, aerial photography is part art and part
> science. No two portraits of the same person look exactly the same.
> Likewise, no two portraits of the earth's surface look the same.

-   **What is Parallax?**

> Parallax is a distortion caused by an aerial image being shot at an
> angle, rather than from exactly above a particular point on the
> ground. More simply, parallax is the apparent shift of an object
> against a background due to a change in the observer's position; the
> apparent motion of an object against a distant background because of a
> perspective shift.
>
> For example, you are driving in the car and you see an oak tree in a
> farmer's field. The tree looks to be directly in front of the
> farmhouse. As you get closer, the tree no longer seems to be in front
> of the farmhouse, rather in front of the barn. Finally you drive past
> the tree and look back. It no longer blocks the farmer's property from
> view; it now blocks your view of another farmer's land you passed a
> mile back.
>
> This same phenomenon occurs with aerial photography, and more so with
> tall objects such as trees or buildings. One year the tall oak tree
> blocks the farmhouse on the imagery, while the next year it blocks
> part of the field, which could lead someone to make the assessment
> that useable acreage for farming had decreased. This could result in
> edits to the Common Land Unit (CLU).

-   **What is Sun Angle?**

> The angle of the sun is a direct result of latitude, time of day and
> year. The sun angle causes shadows to be cast from things at a higher
> elevation onto things at a lower elevation. When shadows are cast, it
> can be difficult to discern what is in the shadow. The area within a
> shadow is (in natural color photography) an area where less data is
> acquired; little light is reflected from within shadows.
>
> Shadows can help identify things, but can also be detrimental. Shadows
> cast by large trees, clouds, buildings, mountains, hills, tops of
> canyons, etc., can hide data that may be required for decision making.
>
> For example, one year the sun was almost overhead, and the trees next
> to a CLU cast almost no shadow. The next year the sun was a bit lower
> in the sky, and the trees seem to have enlarged in size, seemingly
> reducing the acreage of the CLU, and making CLU linework look less
> than perfect. If the shadow is not recognized as a shadow in an image,
> but rather as part of the tree, a change to CLU might be edited with a
> subsequent incorrect determination of acreage.

-   **What is a Digital Elevation Model (DEM)?**

> DEMs are digital files consisting of optimal arrays of point
> elevations, sampled at equally spaced intervals. DEMs are used to
> simulate elevations when creating digital orthorectified photo
> products, or for numerous other purposes when analysis of elevation is
> important.
>
> The accuracy of DEM data depends on the source and resolution of the
> data samples. DEM accuracy is derived by comparing interpolated
> elevations in the DEM with map location elevations. Then a
> root-mean-square error (RMSE) can be computed. RMSE is a measure of
> how closely a data set matches the represented real world elevations.
>
> Although the standard used for many aerial photography acquisition
> projects is the National Elevation Dataset (NED) or a better DEM
> source, there are thousands of different DEMs available. All of these
> DEMs vary in number of points and accuracy. Unless the exact same DEM
> is used, there will be differences in the resulting aerial photography
> product.

-   **What is orthorectification?**

> Orthorectification is the term for the process of removing sources of
> distortion from an image and correcting aerial photography to the
> point where measurements of a feature on the photograph approximate
> ground measurements of the same feature.
>
> Without orthorectification, one would not be able to make measurements
> of distances, positions, or areas from aerial photography.
> Orthorectification tries to remove distortions created by the 3
> dimensional nature of the earth projected into a 2 dimensional
> surface.
>
> Terrain distortion can be reduced by rectifying imagery using a DEM
> and control points. The DEM is used to note mathematical relationships
> between real world topography and the aerial image. These
> relationships are defined, and displacement inherent in the image
> reduced, by warping the image so that distance and area are uniform in
> relation to real world.

-   **What is Aerial Triangulation (AT) and control configuration?**

> Aerial Triangulation is an attempt to identify the exact x, y, z,
> location of an aircraft antenna during exposure. Ground control points
> are used to triangulate signals from the aircraft and help identify
> this exact location. Factoring into AT is the specific
> location/distribution of the control points. Generally, with the use
> of more control points, located closer to the aircraft, the location
> identification will be more accurate. The closer to the real x, y, z,
> location the aerial triangulation calculations are, the more accurate
> the resulting image can be to the true ground location.
>
> There are other methods of pinpointing the exact location of an
> aircraft; airborne GPS and Internal Navigation System (INS) to name a
> few. Depending on the AT solution and control point configuration,
> various levels of known point accuracy will result. If methods and
> resulting accuracies differ from year to year, imagery will look
> slightly different from one year to the next.

-   **What is the difference between absolute and relative control?**

> Horizontal and vertical accuracy of an image when compared to another
> image is considered to be relative accuracy. The two images could be a
> hundred miles from where they actually reside on the earth's surface,
> and still be accurate relative to each other.
>
> With absolute accuracy, an image is not compared to another image, but
> rather to where the data on the image really exists on the earth's
> surface, usually based on a network of control points.
>
> APFO used relative accuracy for NAIP until 2006, comparing new imagery
> to base imagery. After pilot studies in two states, in 2008 seven more
> states will receive imagery terrain-corrected with control points.

-   **What does all this mean to the farmer?**

> Drawing CLU based on imagery is not an exact science. Acreage
> measurements may change slightly when newer imagery is used. However,
> acreage should not change drastically due to replacement imagery,
> unless there was an actual change of land use on the earth's surface,
> such as a new road cut or building on cropland.
>
> Measurements taken from imagery can be extremely accurate and it is a
> huge time saver; however, the *most* accurate measurement of acreage
> still relies on field surveys and GPS.

-   **Who do I contact for more information?**

> 1\. For sales information, contact USDA-FSA-APFO at 2222 W 2300 S,
> Salt Lake City UT, 84119-2020, call 801-844-2922, or visit
> www.apfo.usda.gov.
>
> 2\. If you have general questions about replacement imagery contact
> GIS Specialist, David Davis, at 801-844-2933, or GIS Specialist,
> Louise Mathews, at 801-844-2934.