Crater ages are established by relative freshness of appearance, as indicated by topographic sharpness of their rim crests and degree of preservation of interior and exterior features such as crater floors, walls, and ejecta aprons. Generally, topography appears highly subdued because of the sun angle, and boundaries between map units are not clearly defined. Impact craters larger than about 250 km are referred to as basins. Remnant ejecta blankets around parts of the Beethoven and Raphael basins are subdued in appearance and their margins poorly defined in places. However, where they can be recognized, these extensive aprons allow a generalized regional stratigraphic sequence to be determined.
Line geometry is topologically clean.
Map prepartation was on mylar or acetate made from USGS negatives. Information has been obtained from sources believed to be reliable, but its accuracy and completeness and the opinions based thereon, are not guaranteed. As every effort is made to provide accurate information in this database, USGS would appreciate it if users could call to our attention any errors that may occur by communicating with the contact person.