Elevation Contours

Smoothed contours were produced at 2 foot intervals from topographic vector data (breaklines) collected by photogrammetrists. Breaklines denote the major terrain shifts as percieved by viewing the aerial photography stereoscopically. Major breaks, such as the top and bottom of hills were marked with the breaklines. Point data (DTM) was used to supplant the breakline data to provide enough information to model the terrain of the area. The data was collected at scale of 1"= 40'. Survey field crews surveyed 14 photo identifiable points used for photo control. All the ground control points were used in the final analytical triangulation solution. The horizontal positions were reported in feet; NAD1983 (2011) Massachusetts State Plane Coordinate System, Mainland Zone, Epoch 2010.00. Elevations were based on the NorthAmerican Vertical Datum, 1988. The aerial photographic mission was carried out on April 12, 2017. 459 exposures were taken in 16 flight lines at 3300' AMT resulting in a pixel resolution of 0.22' . The photography was collected with 60% overlap to ensure proper stereo viewing. The digital photographs were triangulated using KLT software. The interior orientations of each photo were measured, the photos were tied togther within flight lines and lastly each flight line was tied, creating one single unified block. This block was then projected into Massachusetts State Plane NAD 83 coordinates using the14 aerial photo ground control points that were collected by traditional survey. RMS formulas were used to compute error propagation and reduce error. The breakline and dtm data collected through the stereocompilation process was edited in KLT Atlas software to check for continuity. A TIN was generated from the edited topographic data which was then used to produce smoothed contours at 2' intervals. The contour information was then checked for errors and converted into AutoCAD .dxf format for GIS import.本数据集的平滑等高线以2英尺为间隔生成，数据源为摄影测量师采集的地形矢量断裂线（breaklines）数据。断裂线用于标示通过立体目视航空影像识别出的主要地形变化，诸如山顶与山谷这类主要地形突变点均通过断裂线标注。数字地形模型（DTM）点数据被用于替代原有断裂线数据，以获取足够信息对研究区域的地形进行建模。本次数据采集比例尺为1英寸=40英尺。外业测量团队共实测14个可通过影像识别的像控点，所有地面控制点均参与最终的解析三角测量解算。平面坐标以英尺为单位，基准为NAD1983(2011)马萨诸塞州平面坐标系大陆带，历元2010.00；高程基准采用1988北美垂直基准面。航空摄影任务于2017年4月12日实施，本次任务在16条航线上共拍摄459张影像，航高为3300英尺（平均海平面以上），最终影像像素分辨率为0.22英尺。影像采集时设置60%的重叠度，以保障良好的立体观测效果。本次数字航空影像采用KLT软件进行三角测量处理：首先测定每张影像的内方位元素，随后在单条航带内完成影像间的连接，最后将所有航带进行整体连接，构建出统一的影像块。随后利用本次外业实测获取的14个像控点，将该影像块投影至NAD83马萨诸塞州平面坐标系中。采用均方根（RMS）公式进行误差传播计算以降低解算误差。通过立体编绘获取的断裂线与DTM数据，在KLT Atlas软件中进行编辑以核查数据连续性。基于编辑完成的地形数据生成不规则三角网（TIN），随后利用该三角网生成间隔为2英尺的平滑等高线。随后对等高线数据进行误差核查，并转换为AutoCAD .dxf格式以用于GIS系统导入。