Understanding sampling interval

The sampling interval controls how the computations are performed when an image is orthorectified or geometrically corrected.

When an image is corrected, OrthoEngine selects a pixel from the output file, computes the elevation from the digital elevation model (DEM), if available, applies the math model to determine which pixel it corresponds to in the raw image, and then transfers the data to the pixel in the output file.

The sampling interval determines how many output pixels are computed following that method. For example, a sampling interval of one (1) means that each output pixel is processed. However, processing every output pixel can take a significant amount of time, and doing so may not be necessary.

To speed up the process, you can increase the sampling interval, which means that OrthoEngine computes the correction for some pixels and interpolates those in between. For example, a sampling interval of four (4) means that the correction of every fourth pixel is calculated and the correction for the pixels in between are interpolated.

Hints for use with orthorectification

When you have high-resolution images, such as aerial photographs, and a high-resolution DEM, a lower Sampling Interval is recommended to ensure that sharp changes in elevation are corrected during orthorectification.
When the resolution of your images is higher than that of your DEM, use a higher Sampling Interval. In this case, many output pixels will fall within the boundaries of a single DEM pixel so using a higher interval speeds up the process without compromising greatly on the output.
If you do not want to use all the detail in your DEM, use a higher Sampling Interval.
If you want to perform a trial run of the orthorectification to verify the results, use a higher Sampling Interval. It will increase the processing speed.

Hints for use with geometric correction

When you are using simple math models, the main advantage of a higher sampling interval is processing speed. Depending on the complexity of the math model, a higher interval likely has less effect on the final accuracy than rigorous math models that use a DEM.