During July and August, of 1890, Professor George Davidson of the United States Coast and Geodetic Survey, together with his assistants, occupied the summit of Mount Conness for the purpose of closing a link in the main triangulation which connected with the transcontinental surveys. Large instruments and much equipment had to be transported to the summit of the mountain by pack animals and upon the shoulders of men. Astronomical observations were made at night, and during the daylight hours horizontal angles were measured on distant peaks in the Coast Ranges from which heliotropes were constantly showing toward Mount Conness. A small square wooden observatory, 8 by 8 feet, housed the 20-inch theodolite mounted upon a concrete pier. Sixteen twisted-wire cables fastened the observatory to the granite mountain top and kept it from being blown away. The officers of the Coast and Geodetic Survey party under Professor Davidson were J. J. Gilbert, Isaac Winston, Fremont Morse, and Frank W. Edmonds. As a result of his own travels and surveys in the region, J. N. LeConte prepared a map of the Sierra adjacent to Yosemite and Hetch Hetchy valleys, which was published by the Sierra Club in 1893, and army officers in charge of park administration did much important map making in the 1890’s. The United States Geological Survey began its mapping of the region embraced within the present park in 1891 and completed the surveys in 1909. R. B. Marshall and H. E. L. Feusier surveyed the Yosemite, Dardanelles, and Mount Lyell sheets; A. H. Sylvester and George R. Davis, the Bridgeport Quadrangle. Operating as they did with limited funds, their efforts spread over a vast territory, and confronted with a short season, they inevitably made some errors on their maps. In correct editions of these maps some ridges, lakes, and canyons have been moved, but today’s travelers may still find lakes and glaciers which are not on the map, and may find a few of these features on the map but not on the ground. It is not the errors of Sierra mapmakers, however, but the measure of success they achieved, which is remarkable. In the higher reaches of the Sierra today it is extremely difficult to discover, after a particularly heavy winter, which snow field conceals a lake and which covers merely a meadow or an expanse of ice. Nor is the Sierra itself utterly static. At least two small lakes which formed behind dams of glacial moraine have disappeared recently when the dams were undermined.