Loudoun Imagery 1992 (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"劳登县多年来持续收集航空影像数据。自2000年代中期起，影像均以数字格式采集，而更早年份的影像则采用胶片相机拍摄。此前年份的胶片卷以胶片罐形式保存于该县档案库中，但无法在现代数字地理信息系统（GIS, Geographic Information System）中调取查看。为此，该县启动专项项目，将这批胶片卷扫描为数字格式。随后，项目团队采用现代摄影测量技术对扫描后的胶片进行处理，生成劳登县的数字正射影像镶嵌图。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）图像，其余年份的影像均为全色（黑白）图像。MrSID镶嵌影像采用50:1的压缩比。包含扫描文件名属性的近似接缝线位置数据也已一并提供。本次航空影像基本覆盖弗吉尼亚州劳登县的全部陆地区域。由于影像采集时的时间环境与地面条件存在差异，各影像的色调、质量及平面位置精度会存在一定差异。本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。