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<br />, <br /> <br />either side. The maps shall be screened positives <br />on 4 mil mylar base material using DuPont Cronaflex <br />or an approved equivalent. A screen density of 133 <br />lines per inch shall be used for the final scale of <br />the map produced. If 2 scales are produced, each <br />scaled final positive shall be 133 lines per inch. <br />Variance in tone and/or contrast between splice <br />lines of photo negatives shall not be objectiona11y <br />noticeable to the eye. More than 4 tone variances <br />per final mylar map shall be considered unaccept- <br />able. At the printing scale, the contour lines, <br />names, and lettering for the title shall be of such <br />size that, when photographic prints of the photo- <br />map are made to the size and scale required, indi- <br />vidual letters shall be not less than 0.1" in <br />height and shall be clearly legible. All overlay <br />lines and lettering shall be black or as specified <br />by the contracting agency or its representative. <br /> <br />2. Conventional Rectification Method. By this method, <br />the contractor shall produce a photographic back- <br />ground with contours by correcting for the effects <br />of tilt and relief prior to pasting down the photos <br />on a base network of horizontal control. These <br />photo-maps have been found to be fairly accurate in <br />flat terrain. The contractor shall obtain the best <br />contour match near the stream reaches. For the <br />purpose of this specification, it shall be consid- <br />ered that conventional rectified photo-maps with <br />contours will not meet "National Map Accuracy <br />Standards." Therefore, t.o meet the accuracy <br />requirement of the contract, the contractor shall <br />provide a line map showing contours and topographic <br />features at no extra cost. No photo-maps shall be <br />acceptable with rectified enlargements greater than <br />5 diameters from the flight photography. <br /> <br />3. Or tho ona1 pro'ection (Ortho hoto) Method. In this <br />process, differential incremental elements of <br />photographs are photogrammetrica1ly repositioned <br />with the aid of an orthophoto scope (ortho- <br />scanning). This is a linear incremental transfor- <br />mation that results in the scale of the stereo <br />model being uniform throughout for the mapping <br />area. Each stereo model shall be scanned with a <br />slit width and at a speed which will achieve uni- <br />form tone and aesthetic and accurate mapping. <br />During scanning, the floating mark shall generally <br />be kept in contact with the ground, even to the <br /> <br />-5- <br />