east and northwest corners become 60 degrees in the projection. All north-south 

 and east-west lines are parallel, and scaled distances between map points along 

 these parallel lines are the same as on the map. Scaled distances in any direction 

 except parallel to these lines are either greater or less than those on the map. 

 Obviously then, in order to transfer map information to the projection, it is 

 necessary to do so by means of coordinate measurement. Point a on the map 

 is 10.5 units (any scale) east of the northwest corner of the map and 2.8 units 

 south. Point a on the projection is the some number of units along corresponding 

 lines. Any point on the map can be accurately located on the projection by 

 scaling along coordinates; and this procedure must be followed in locating a 

 sufficient number of control points, such as the confluences of streams, road 

 intersections, and well locations, to insure accurate sketching between these 

 points. 



When the wells are located on the projection, logs are plotted at the locations 

 on a vertical scale adapted to the scale of the projection so that the desired 

 effect is attained. The isometric base is considered as a level plane. Therefore, 

 if it is assumed to be at sea level and the logs are placed so that the sea-level 

 point on the log is adjacent to the map location, then the panels will represent 

 the structure. Of course, if the bottoms of the wells are above sea level, then 

 the plane of the projection should be at some datum high enough to cut 

 all the wells. 



The plane of the projection may also be considered a stratigraphic datum, 

 i.e., the top of a formation; and all the logs are then placed with this horizon 

 at the location point. Another common practice is to draw subsurface geology 

 entirely below the plane of the projection and surface geology above. The 

 principal objection to this method is that the drawing may have neither a 

 structural nor a stratigraphic datum. The panel projection in Figure 24-525 

 is drawn with the plane at the top of a formation; this, therefore, is a strati- 

 graphic projection. The panels have not been completed in order to permit a 

 clearer view of some of the map features. 



Block Diagrams and Other Illustrations 



It is of the utmost importance that the geologic concepts developed as a 

 result of studies in structure or stratigraphy be shown in some manner that is 

 most comprehensible to those who have only occasional contact with the projects. 

 Maps and cross sections sometimes fail in their purpose of conveying to others 

 certain complex geologic conditions, mainly for the reason that each is two- 

 dimensional — one in the horizontal plane, the other in the vertical. Block dia- 

 grams effectively combine the features of both maps and sections and are, 

 therefore, an indispensable mode of illustration. 



Geologic block diagrams are constructed according to certain principles of 

 projection and perspective. Space here does not permit going into all the de- 



515 



