These medium resolution true color images represent the first color aerial photo "basemap" for the Commonwealth from MassGIS. MassGIS (then part of the Executive Office of Environmental Affairs) and the Massachusetts Dept. of Transportation (then the Mass Highway Department) jointly funded the project.
The photography for the mainland was captured in April 2001 when deciduous trees were mostly bare and the ground was generally free of snow. Photography for the Elizabeth Islands, Marthas Vineyard, and Nantucket (the "Islands") was captured in April 2003.
Keystone Aerial Surveys Inc. (KAS) of Philadelphia, PA acquired the aerial photography for the entire project using Kodak AeroColor 2444 and AeroColor 2445 film. KAS used large format Leica RC-30 calibrated aerial mapping cameras employing forward motion compensation and gyro-stabilized mounts. Each camera was also equipped with a tie to the aircraft GPS receiver. The flying altitude was approximately 15,000 feet to provide an original photo scale of 1” = 2,500’. Endlap (forward overlap) was 60%, except 80% in areas with tall structures, with (high) sidelap of 42%.
The processing of the photos was conducted by two firms. Intermap Technologies of Englewood, Colorado and Ottawa and Calgary, Canada processed the mainland portion of the Commonwealth east of the 165000-meter line (in MA State Plane coordinates) and Chas. H. Sells, Inc. of
Charlton, MA processed the remaining area west of this line (approximately 72 degrees longitude), as well as the Islands. These vendors provided scanning, softcopy aerial triangulation, orthorectification, image mosaicking and quality control.
Ground and photogrammetric control (elevation model) for the orthorectification of the aerial photography was available statewide from the previous black-and-white 1:30,000 digital orthophotography program conducted from 1992-2000 (see the 1990s Aerial Imagery description). The transfer of control from that project made the creation of these new data much more efficient, and assured a very high level of accuracy and compatibility.
The color negatives were scanned at 14 (12.5 Sells) microns and then inverted. The pixels were subsequently resampled to 0.5 meters ground resolution. Each pixel is coded with three 8-bit values representing the amounts of Red, Green, and Blue in the pixel. This produces a 24-bit RGB image. 8-bit (values ranging 0-255) Red, Green, and Blue values are packed into a single 24-bit identity. This allows for a maximum of 16,777,216 unique combinations of colors.
The imagery was radiometrically balanced prior to mosaicking. Only minor adjustments were made to the brightness and contrast. In the Intermap data, to avoid color distortion of land areas, color balancing was preceded by the removal of data in large bodies of water and the ocean. The water pixels were then merged back in. Since the Sells data did not contain large bodies of water (except for the Quabbin Reservoir), it was not processed in this manner.
The original 0.5 meter data was delivered to MassGIS in GeoTIFF format in orthoquad tiles representing 4 x 4 km on the ground (8000 rows x 8000 columns, equaling 64,000,000 pixels). Since each pixel contains 24 bits (3 bytes) of data, a full resolution image covering one orthoquad tile is approximately 192,000,000 bytes (183 Megabytes) in size. The tiles are based on the Orthophoto Index Grid and are named <sheet-id>.tif, where the <sheet-id> is the first 3 digits of the state plane XY coordinate pair for the lower right corner of each cell. Header files (named <sheet-id>.tfw) were also delivered for use in some GIS software. Imagery is georeferenced to Massachusetts State Plane (Lambert Conformal Conic Projection) NAD83 denominated in meters.
BSC Group fulfilled land survey requirements by performing highly precise ground control survey & independent verification and validation of spatial accuracy.
Displaying the Images
Users should note that color tone and balance are always based on personal preference. Image color processing frequently involves a series of compromises with the overall objective of providing images “with the appearance that people expect”. Different hardware (monitor, video card, etc.), software, and subjective perceptual differences can all contribute to the perception of color in these images. Users should also note that visual differences may be apparent between the eastern and western images due to slightly different processing methods by the two vendors.
Printing or plotting the images further introduces variability due to different hardware and software systems, etc. Therefore MassGIS is distributing the imagery as processed by our vendors and leaving additional enhancement (e.g., contrast and brightness adjustment, etc.) to the user. For example, ArcGIS 8.x users may want to set the display properties for these images to "Resample during display using Cubic Convolution (for continuous data)" in the Properties box, Display tab. (The Cubic display method may result in gaps between adjacent images; ESRI is working to fix this). Other options include adjusting the Contrast and Brightness settings on the Effects Toolbar or applying a stretch (e.g. Standard Deviation) to the images in the Properties box, Symbology tab. Achieving results you like may take exploring the capabilities of your software and hardware, which can take considerable time.
Displaying with Feet-based data
Users may successfully display our meters-based imagery in ArcView 3.x with other data in NAD83 Mass. State Plane feet simply by changing the accompanying .sdw files and installing the newest version of the file AVMrSID.dll.
The process is as follows:
1 - Open the .sdw file in a text editor and multiply all values by 3.2808333316 and save
2 - Place the AVMrSID.dll file in the BIN32 folder where ArcView is installed (e.g., C:\ESRI\AV_GIS30\ARCVIEW\BIN32)
If users wish to display the images with feet-based (or any) data in ArcMap, load the image file (.sid) and the companion .aux file and the data (if it has a .prj file, indicating a defined spatial reference for vector data) and the images will project on the fly (ArcMap does not use the .sdw).
Please note that MassGIS does not have any old aerial photographs (pre-1992).
The MassDOT Survey Section maintains an archive of aerial photographs dating back to the 1930’s