The reported altitude of the surface and each mandatory level is written on the right side of the colored portion of the diagram immediately above the printed isobar value (fig. 4-33). Notice that the 925-hPa mandatory level is not printed on the diagram. Mark the 925-hPa level with a tic mark on the edge of the diagram and enter the reported altitude just above this level, as shown in the figure. The three digits encoded for the altitude are converted to actual altitude values by the observer, following the guidance given in table 4-2. When the reported surface pressure is less than 1,000 hPa, such as 976 hPa, an extrapolated 1,000-hPa height below station level is reported following the 00 indicator for the 1,000-hPa level. (In this case the 1,000-hPa level temperature/dew-point depression group and the winds group would be encoded as slashes.) For example, a reported level 00531 ///// ///// indicates a 1,000-hPa height as -03 1 meters (the 5 indicates a negative value), while a reported height of 00631 would indicate -13 1 meters. The height of the surface level is assumed to be zero. The height is then plotted as a small dot surrounded by a 2/8- to 3/8-inch square, at the intersection of the isobar of the mandatory pressure level and the isoheight line of the constructed height scale (fig. 4-34). Each printed isotherm is 100 meters. Estimate placement of the dot to the nearest 10 meters. On the same horizontal isobar, the temperature is plotted as a small point surrounded by a 2/8- to 3/8-inch- diameter circle where the isotherm crosses the isobar. Temperatures are plotted to the nearest tenth degree Celsius (fig. 4-35). Estimate the position between the printed  isotherms.  Remember,  if  the  reported  tenths value is even, the temperature is above zero, and if the tenths value is odd, the temperature is below zero degrees  Celsius. To plot the dew-point depression, move along the isobar toward the left the number of degrees and tenths of  degrees  indicated  by  the  encoded  dew-point depression. Place a small point at the indicated value and surround the point with a 2/8- to 3/8-inch-high triangle (fig. 4-36). Remember, dew-point depression is the difference between the dew-point temperature and the temperature, and the dew-point temperature is either the same as or lower than the temperature. Also, remember that dew-point depression values are in degrees and tenths of a degree Celsius for code figures 00 to 50, while reported values 56 to 99 are in whole degrees when 50 is subtracted from the code figure. For example, the code figure 43 is a dew-point depression of 4.3°C, while 63 is (63 - 50 = 13) a dew-point depression of  13.0°C. The next encoded value, wind direction, is plotted on the right wind scale for the first sounding plotted on a diagram, and succeeding reports are plotted on the Table 4-2.—Rules for Converting Encoded Altitude to Actual Altitude LEVEL  (hPa) 1000 925 RULE no  change 850 prefix with a 1 700 500 400 300 prefix with a 2 if 500, or a 3 if <500 Suffix with a 0 250 200 150 100 70 prefix with a 1 and suffix with a 0 50 prefix with a 2 and 30 suffix with a 0 20 Figure 4-33.—Plotted altitude values. 4-30