Loudoun Imagery 1994 (BW)

Loudoun County has collected aerial imagery for many years. While imagery has been captured in digital format since the mid-2000s, prior years were captured with film cameras. Rolls of film from previous years were available at the County in film canisters, but unavailable for viewing on modern digital GIS systems. The County initiated a project to have these film rolls scanned into a digital format. The film was then processed using modern photogrammetric techniques to produce a digital orthomosaic of the County. Images from 1979, 1986, 1988, 1990-2007 were processed and delivered as tiled uncompressed GeoTIFF and MrSID mosaic formats. 1979, 1986, and 1988 were captured and produced as color (R,G,B) images, while the remainder were captured and produced as panchromatic (B/W) images. The MrSID mosaics use a 50:1 compression ratio. Approximate seamline locations containing the scan name attribute have also been provided. The aerial imagery generally encompasses the full land area of the County of Loudoun, VA. Images were flown in varying temporal and ground conditions and will exhibit some variation in tone, quality, and positional accuracy. "Digital Orthophotography compiled by Sanborn Map Company, Inc. Colorado Springs, CO"弗吉尼亚州劳登县（Loudoun County）多年来持续收集航空影像数据。自2000年代中期起，影像采集均采用数字格式，但更早年份的影像均通过胶片相机拍摄。早年拍摄的胶片卷以胶片罐形式存档于该县档案馆，但无法在现代数字地理信息系统（GIS, Geographic Information System）中直接查看。该县启动专项项目，将这批胶片卷扫描为数字格式。随后采用现代摄影测量技术对扫描影像进行处理，生成该县的数字正射影像镶嵌图（digital orthomosaic）。1979年、1986年、1988年以及1990年至2007年的影像均完成处理，并以分幅未压缩GeoTIFF和MrSID镶嵌格式交付。其中1979年、1986年及1988年的影像为彩色（RGB，红、绿、蓝）影像，其余年份的影像均为全色（黑白）影像。MrSID镶嵌影像采用50:1的压缩比。数据集同时提供了包含扫描文件名属性的近似接缝线位置信息。该航空影像数据集基本覆盖弗吉尼亚州劳登县的全部陆地范围。由于影像采集时的时间条件与地面环境存在差异，影像的色调、质量及位置精度会存在一定波动。“数字正射影像由科罗拉多州科罗拉多斯普林斯市的桑伯恩地图公司（Sanborn Map Company, Inc.）编制”