SUBCOURSE EDITION - Modern Prepper

. To plot the geographic coordinate references, the draftsman must know the grid distances(east or west and north or south) from a major grid intersection to the geographic coordinate. Thetransformation of geographic coordinates into grid coordinates is a topographic computing function.Tables have been compiled giving the grid coordinates for geographic intersections at even 5-minuteintervals for the five spheroids (Clarke 1866, Clarke 1880, International, Bessel and Everest).In addition, tables for intersections at 7 1/2-minute intervals have been prepared for the Clarke 1866spheroid. (Refer to appendix A for complete listing of appropriate manuals). The grid coordinates inthe tables are given to 0.1 meter which is more than adequate to meet the accuracy requirements.c. All the information necessary for plotting the corners of a given sheet is provided to the mapcompiler on DA Form 1941, Grid and Declination Computations, prepared by the topographic computerin accordance with instructions in TM 5-237. A sample of this form is shown in figure 4-6. From thefurnished computations, the compiler can determine the central meridian of the grid zone in which thesheet falls, the exact grid coordinates of each of the four corners, a measurement in centimeters fromthe first full grid line in each direction to the corner, the conversion factor to be used in converting theground measurement of grid distances to map measurements, the overall measurement, in centimeters ofeach of the four neat lines, and grid and magnetic declination data for the sheet.(1) Before plotting the corners, it is important to determine exactly the grid squares in whichthe corners will fall, and to locate them in such a way that the sheet will fall entirely within the area ofplotted grid lines. Since the grid lines on the form are the first full lines on the sheet, partial lines arenot indicated or labeled. If the measurement to the corner is greater than 2 centimeters (at 1:50,000scale), there is a partial grid line between the one shown on the form and the actual corner. Be sure toallow for this, and then label carefully those grid lines to be used in the plotting.(2) The actual plotting of the corners is a simple mechanical operation of setting thecentimeter measurement on the dividers from the Invar scale, and plotting this amount in the directionsindicated on the form. The intersection of the two plotted values is the sheet corner.d. The east and west neat lines on either side of the sheet are easily obtained by simplyconnecting plotted corner intersections with straight lines. The north and south neat lines, however,which represent parallels of latitude, are curved lines, and require further computations. To achieve thenecessary curvature, grid coordinates of the five-minute intersections along the top and bottom of thesheet are computed from the tables of coordinates for 5-minute intersections for the appropriatespheroid. The Index to Spheroids in TM 5-241-1 specifies the correct spheroid for each area of theworld. The 5-minute intersections are plotted in the same way as the corner intersections. The 5-minute intersections in the interior of the sheet must also be computed and plotted. The procedure forobtaining the necessary grid values for the 5-minute intersections is on the next page.137