Stereoscopic aerial photography
with 80% forward and 40% side overlap was collected along flight lines
running approximately north/south during spring "leaves off" periods at
a flying height of 15,000 ft. with a 6 inch mapping camera with forward
motion compensation. The scale of the photography is therefore 1:30,000.
Ground control targets were set out
prior to the flight and photo recognizable points were substituted for
targets that were lost. Horizontal control (referenced to NAD83) conforms
to the Federal Geodetic
Control Committee specifications for Second Order Class 2 GPS surveys.
Vertical control (referenced to NAVD88) is within 10 cm. It is tied to
second order Class II NGRS benchmarks. In addition the latitude, longitude
and ellipsoidal heights of continuously operating GPS reference stations
were incorporated into the control adjustment. Orthometric heights were
directly measured or estimated from GPS derived ellipsoidal heights.
Aerial Triangulation (AT) block models
were developed and tested for accuracy. Adjacent blocks are tied to new
ones to insure a "seamless" image. These reports are on file at MassGIS.
DTM data points were collected on
analytical stereoplotters at a sufficient density to support generation
of 3 meter contours conforming to the National Map Accuracy Standards (+
or - 1.5 meters). Mass points were collected along parallel scan lines
75 meters apart at variable density as a function of the topography and
other ground features. Spot elevations at summits and in depressions and
breaklines along significant linear features were also collected. Distinctions
between "hard" and "soft" breaklines were established and standardized
to facilitate the generation of contours.
The photography was scanned at 15
microns and the images were differentially rectified using the DTMs and
the AT block models. The histogram for tonal adjustment provides for a
range of gray shades from 30 to 225 (out of 256), thus allowing pure black
and white to be legible when over plotted on the images. Accuracy of the
image was given precedence over tonal consistency at the edges of the images.
The final digital images were clipped with the Orthophoto Index Grid, thus
the tiles do not overlap. The images meet or exceed the National Map Accuracy
Standards to the extent that 90% of the well defined features fall within
0.5mm of their true position on the ground at the nominal output scale
of 1:5,000 (2.5m on the ground). Additionally, the maximum displacement
of well defined features is less than 5 meters. Each pixel in the digital
orthophoto image represents 0.5 meters on the ground.
Each tile contains 8,000 x 8,000
pixels which equates with a file of 64 megabytes (mb). These images have
been resampled at 1 (16 mb), 2 (4 mb) and 5 (640 kb) meter resolutions.
The half-meter images are stored in .BIL format with associated .HDR files.
The resampled images are in tiff format with an associated .tfw world file
for georeferencing. The resampling process was performed in the ARC/INFO
GRID module. MassGIS also distributes the half-meter and one-meter
orthos in the MrSID format as individual tiles, and the one-meter orthos as 10
large MrSID mosaics.
All image acquisition and processing was performed by Intermap Technologies.