Each
GeoCover mosaic image covers an area approximately six degrees of latitude by
six degrees of longitude.
To completely cover a state and the Landsat scene footprints around a
state, multiple GeoCover mosaics are used.
The
GeoCover handling steps are 1) Imported
into Erdas Imagine 2) Reduced to band two only 3) Resampled from 28.5 to 30
meters 4) Reprojected to one common UTM
zone per state 5) Stitched together with other images to cover the state
footprint 6) Exported to .LAN format.
All
CDL products are currently processed entirely by PEDITOR
Finalized mosaics are exported to Geotiff format upon completion.
PEDITORs
Batch program routines process the scenes in as little as a few hours for the
simple mosaics, and can run up to 2/3 a day on a large/complex state with many seams/stitch lines/cloud
problems.
The
Landsat TM/ETM+ scenes used are radiometrically and systematically
corrected. There is a need to tie down
registration points on a continuing basis
for every scene within each state in the project. Without some image/image registration, the
scene registration tends to float 2-3
pixels in any given direction, for any given scene.
An automated registration method was developed to co-register each raw
scene to an ortho-base image, and then to apply each raw scenes correlation coefficients to the categorized scenes.
Image
recoding is necessary between different analysis districts, to rectify to a
common signature set for a state.
Once an Analysis District is categorized, it possess a unique set of
signatures. These signatures are
recoded to a master signature set. For instance, a corn signature or category
may have between 1 and 50 classes for a given Analysis District, and they are condensed into one class or digital number
for the final mosaic. This is done for
every cover type in the project.
Clouds
pose a big problem when trying to make acreage estimates, and there are
mechanisms within PEDITOR to minimize their extent, as there are ways to minimize cloud coverage in the mosaic
process by prioritizing scene overlap.
Each
categorized scene needs to be geo-registered to an ortho-base image. A block
correlation is run between band two from each raw scene, and band two of the ortho-base image. The registration
of the GeoCover mosaicked scene and the individual raw input scenes are used to get
an approximate correspondence. A
correlation procedure is used on the raw Landsat scenes and the mosaicked
scene to get an exact mapping of each pixel from the input Landsat
scenes to the mosaicked scene. The
results of the correlation are used to remap the pixels from the individual input
scenes into the coordinate system of the mosaicked scene.
The
mosaic process now performs: 1) Automates image registration/rectification,
2) Converts each categorized image and associated
statistics file to a set standard automatically (recode), 3)
Specifies overlap priority by scene or county, 4) Filters out clouds
when possible
Each scene is co-registered to EarthSat's
GeoCover LC imagery (50 meters RMS), and then stitched together using the
priorities previously assigned from the scene observation dates/analysis
districts map.
Scenes/analysis districts with better quality
observation dates are assigned a higher priority when stitching the images together.
Clouds are assigned a null value on all
scenes, and scenes of lower priority that are cloud free, take precedence over
cloudy
higher priority images.
Once cloud cover is established throughout
the mosaic the clouds are assigned a digital value.