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With maps displayed on a computer screen it sometimes seems a bit odd to speak of a certain map as having a specific scale.  Naturally, the scale of the displayed image will depend on the zoom level and the size of your monitor.  So the three scales mentioned in the right-button menu refer to the scale of the source material from which the DLG surfaces were created.  (Sometimes xviewsdts and this document refer, more properly, to the map series of a surface rather than its scale.)  Each of the three map scales supported by xviewsdts can be displayed at any zoom level.  For our purposes the different scales simply indicate the level of detail and accuracy with which entities are located on the surface.  Large scale surfaces contain more detail and are more precise than small scale surfaces.  The trade off is that large scale surfaces generally cover less area than small scale surfaces.  1

The following sequence of images demonstrates loading surfaces from different scales of source maps which cover the same area, and also loading adjacent surfaces from source maps of the same scale.  (Although this example shows a progression from small scale, through intermediate scale, to large scale, surfaces may be loaded in any order.)

Here is a small scale surface showing the hydrography theme for an area south-central Massachusetts and northeastern Connecticut.  Notice that there is little detail, and that the shapes of the water bodies are fairly simple.  To view an intermediate scale surface of the same area, position the pointer at your point of interest. . .



Then bring up the right-button menu and select Load Intermediate Scale.



The intermediate surface is loaded, showing more detail, and more complexly defined shapes.  To view a large scale surface of the same area, position the pointer at your point of interest. . .



Then bring up the right-button menu and choose Load Large Scale.



The large scale surface is loaded.  Now there is a lot of detail.



Notice that because this surface covers a smaller area than did the intermediate scale surface, the area on the right side of the window is blank.  To load the surface for that area, position the pointer in that area. . .



Then bring up the right-button menu and again choose Load Large Scale.



The adjacent surface is loaded.



 


1 Generally, Large scale surfaces cover the full 7.5 x 7.5 minute area as the source map.  Intermediate scale surfaces generally cover only 15 x 15 minutes, one-eighth of the area covered by their source map.  In urban areas some themes (especially Roads & Trails) may only cover 7.5 x 7.5 minutes.  Small scale maps generally cover an entire state or a group of states.

Minutes of latitude & longitude covered by one surface of various scales
Large scale Intermediate scale,
rural areas		 Intermediate scale,
roads & trails theme in urban areas		 Small scale
7.5 x 7.5 15 x 15 7.5 x 7.5 Entire state


Copyright © 2002, 2005 by Dapper Mapper

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