Groton, Massachusetts 15 Minute Digital Raster Graphic

This layer is a digital raster graphic of the historic 15-minute USGS topographic map of the Groton, Massachusetts quadrangle. The survey date (ground condition) of the original paper map is 1887, the edition date is October, 1893 and the map was reprinted in January, 1910. A digital raster graphic (DRG) is a scanned image of a U.S. Geological Survey (USGS) standard series topographic map, including all map collar information. The image inside the map neatline is geo-referenced to the surface of the earth and fit to the Universal Transverse Mercator projection. The horizontal positional accuracy and datum of the DRG matches the accuracy and datum of the source map. The names of quadrangles which border this one appear on the map collar in their respective positions (N,S,E,W) in relation to this map.

One of the most widely used of all maps is the topographic map. The feature that most distinguishes topographic maps from maps of other types is the use of contour lines to portray the shape and elevation of the land. Topographic maps render the three-dimensional ups and downs of the terrain on a two-dimensional surface. Topographic maps usually portray both natural and manmade features. They show and name works of nature including mountains, valleys, plains, lakes, rivers, and vegetation. They also identify the principal works of humans, such as roads, boundaries, transmission lines, and major buildings. The wide range of information provided by topographic maps make them extremely useful to professional and recreational map users alike. Topographic maps are used for engineering, energy exploration, natural resource conservation, environmental management, public works design, commercial and residential planning, urban planning and design, and outdoor activities like hiking, camping, and fishing. These maps can provide an excellent view of the changes that have occurred in the cultural and physical landscape. As this map has been geo-referenced, it may serve as a useful basemap in conjunction with other GIS vector data.

The Harvard Map Collection owns a near-complete set of the 1:62,500 scale 15-minute USGS Topographic Quadrangle series maps. These maps were produced between the early 1880s to the mid-1960s. The Map Collection also holds a collection of 30-minute (1:125,000) historical (1882-1940) USGS maps. Produced at a scale of 1:62,500 (some maps are produced at a scale of 1:63,360), these maps are commonly known as 15-minute quadrangle maps because each map covers a four-sided area of 15 minutes of latitude and 15 minutes of longitude. The United States has been systematically divided into precisely measured quadrangles, and adjacent maps can be combined to form a single large map. Each quadrangle is designated by the name of a city, town or prominent natural feature within it, and the titles of adjoining quadrangles are printed on the margins of each map. The Map Collection also owns numerous other USGS topographic maps in other series with various scales, published earlier in the 20th century (before the 7.5 minute series dominated the work of the USGS) as well as USGS topographic maps published in the late 19th century.

ground condition.

raster digital data.

None.

The DRG is an 24-bit color image that employs a color palette to ensure uniform colors throughout a particular DRG series. All DRGs within a series must have the same RGB value. The DRG is a faithfully reproduced digital image of the original source map. Some differences may be detected between the source graphic used and the DRG due to the RGB values assigned that particular color. The intent is to recreate those colors as near as possible. Data completeness for DRG files reflect content of the source graphic. Nearly all of the maps have an "edition" date and one or more survey and revision dates. Use the survey and revision dates to determine the vintage of the cartography. These dates are usually found in a map's lower left corner. Edition dates (in the lower right corner) can be confusing, because over time, the USGS has changed the method by which edition dates are assigned. For the earliest maps in the 1880s, new edition dates were assigned nearly every time the map was reprinted. In later years, the original edition date was retained for each subsequent reprinting. Such dates may be assumed to roughly represent the year that the map was first published. The original USGS quadrangle paper maps in this series were produced before the National Map Accuracy Standards were developed in 1941. These standards were developed to ensure that Federal Government maps meet the high expectations and requirements of users. These standards require horizontal and vertical map precision; these and other standards of accuracy and content ensure consistency in both the detail and the appearance of maps. They also ensure compatibility among USGS maps made at different times. The positional accuracy of the original map was largely preserved during the digitizing and geo-referenceing processes. Accuracy may be generally characterized as posessing the highest standards of accuracy given the general survey, instrumentation, design and printing technologies and cartographic techniques available during the time the original map was produced. The original map was scanned and geo-referenced (SEE: Process Steps) to produce this digital version. While the data of the published map is retained in the georeferenced image, the image is referenced to the Universal Transverse Mercator coordinate system, which is inconsistent with the original projection of the paper map, and may lead to discrepancies in the horizontal accuracy of the DRG. The standard used for georeferencing the image to the UTM coordinates was an RMS error below 50 meters. See the process steps for the actual RMS error achieved. Refer to the DRG collar for information about vertical positional accuracy.

Microsoft Windows 2000 Version 5.0 (Build 2195) Service Pack 4; ESRI ArcCatalog 9.0.0.535.