WILD 5750/6750

LAB 510/04/2010

IMAGE MOSAICKING

MOSAICKING:

The MOSAIC Imagine tool

The Imagine MOSAIC toolOften, a study area will

fall across two raster data sets allowing for the need to mosaic (or merge) data sets together.

The Imagine MOSAIC tool

The Mosaic tool allows us to merge multiple raster data sets (of the same type) such as satellite imagery, aerial photography, or Digital Elevation Models into one continuous file.

Several issues must be considered when mosaicking images together:

•Select a “base” image”

•Resampling Method

•Same map projection

•Mosaic Method (overlap function)

•Need for image balancing or color corrections

The Imagine MOSAIC tool

The Imagine MOSAIC tool

Resampling Method

Resampling Method: As discussed in class, each resampling method utilizes a different data sampling algorithm. Ensure that you select the proper method for your study.

The Imagine MOSAIC tool

Mosaic Method (overlap function)

Using the EDIT SET OVERLAP FUNCTION menu, the user can specify the data overlap function desired.•Overlay: The overlap area belongs to the last image opened, which is on top in the stacking order. Often, for analysis, this is the preferred overlap method.

•Average: The value of each pixel in the overlap area is replaced by the average of the values of the corresponding pixels in the overlapping images.

•Minimum: The value of each pixel in the overlap area is replaced by the lesser value of the corresponding pixels in the overlapping images.

•Maximum: The value of each pixel in the overlap area is replaced by the greater value of the corresponding pixels in the overlapping images.

•Feather: The overlap area is replaced by a linear interpolation of the pixels in the overlap. A pixel in the middle of the overlap area is 50% of each of the corresponding pixels in the overlapping images. A pixel 1/10 of the overlap from an edge would be 90% one image and 10% the other.

The Imagine MOSAIC tool

Color Corrections

Using the Histogram Matching:

-extract the histogram of the base image and use and algorithm to extrapolate it to the other image(s)

The Imagine MOSAIC tool

Other Details

•Note that images being mosaiced must be in the same projection.

•Numerous file input types may be used (i.e. jpeg, tiff, dem, grid, img), however, the output mosaiced image will always be .img (ERDAS Imagine Format).

•Size and processing time should be considered when merging to high resolution data sets.

•If images were georeferenced with different accuracies, there may be feature (i.e. road, stream, agriculture) offsets across the image cutline.

The Imagine MOSAIC tool

The Process•From the EDIT ADD IMAGES menu, select the images you wish to mosaic.

•Determine the proper overlap function, resample method, and if color balancing is needed.

•If using the OVERLAP method as your overlap function, determine what image should fall on top of the others. This is done using the SEND SELECTED IMAGES buttons on the toolbar.

•From the PROCESS RUN MOSAIC menu, start the mosaic process. The user will be prompted for a Output File Name and Location.

The Imagine MOSAIC tool

The Process

AssignmentUse the images supplied in the Lab5 folder, and the

instructions contained in the tutorial “Lab5_Mosaic_TourGuide.pdf”.

1. Provide a screen shot of your resulting image (mosaic) for the exercise that uses air photos. (a) Why did you have to use a cutline? (b) Why do you think that it is best to use linear landmarks (rivers or roads) when drawing cutlines?

2. Provide a screen shot of your resulting image (mosaic) for the exercise that uses Landsat scenes. (a) Why do you think that a color correction (specifically histogram matching) was applied? Now, other scenario…assume that you had applied a radiometric correction (meaning that instead of raw digital numbers now you have reflectance values) to these images prior to mosaicking… would you still use the color correction (histogram matching)? Why?

3. Provide a screen shot of your resulting image (mosaic) for the exercise that uses color infrared photos. Why do you think that a color correction (color balancing) was applied?

Note: This lab assignment is due on October 11th, 2010. Email your work to: doug.ramsey@usu.edu