In the sexagesimal or North American system, the circle is divided into 360 equal parts known as DEGREES of arc, each degree into 60 equal parts known as MINUTES of arc, and each minute into 60 equal parts known as SECONDS of arc. As an example, angles in this system are written as 26304716.48 which is read as "two hundred sixty-three degrees, forty-seven minutes, and sixteen point four eight seconds of arc. " In the United States, this is the most commonly used system of angular measurement.

In the centisimal or metric system, the full circle is divided into four quadrants, and each quadrant is divided into 100 equal parts known as GRADS or GRADES. Each grad is further divided into decimal parts. As an example, angles in this system are written as 376^g .7289, or 376^g 7289" which is read as "three hundred seventy-six point seven two eight nine grads," or as "three hundred seventy-six grads, seventy-two centisimal minutes, and eighty-nine centisimal seconds. "

In the mil system, the circle is divided into 6,400 equal parts known as MILS. The mil is divided into decimal parts. As an example, angles in this system are written as 1728.49 roils, which is read as "one thousand seven hundred twenty-eight point four nine mils." This system is used principally by the artillery people. The significance of this unit of angular measure is the fact that 1 mil is the angle that will subtend 1 yd at a range of 1,000 yd.

The relationship among values in the three systems of angular measure are as follows:

Aside from the units of measurement discussed above, the EA must also deal with other units of measurement, such as TIME, TEMPERATURE, PRESSURE, and so forth. He must use exact time in computing problems in astronomy and some laboratory works. He must be able to apply temperature corrections to his tape readings. He must also evaluate the effect of atmospheric pressure at different elevations and get involved in some other types of measure that will be discussed in the following paragraphs.

For practical purposes in everyday affairs and in surveying, the measurement of time intervals is of great concern. The time used in everyday life is known as STANDARD TIME and is based on the mean apparent revolution of the sun around the earth because of the earths rotation on its axis. Standard time is used in surveying to regulate the normal days operations. But, when it is necessary to observe the sun or the stars to determine the azimuth of a line or the position of a point on the earths surface, the surveyor uses three other kinds of time. They are APPARENT (true) SOLAR TIME, CIVIL (mean solar) TIME, and SIDEREAL (star) TIME. You will learn more about these different times when you study the chapter on "Geodesy and Field Astronomy" in Engineering Aid 1 & C.

In all four kinds of time, the basic units of measure are the YEAR, DAY, HOUR, MINUTE, and SECOND of time. The duration of any one of these units is not the same for all kinds of time. For example, the sidereal day is approximately 4 min shorter than a standard- or civil-time day. In the practice of surveying, it is customary to say, or write, the time of day as the number of hours, minutes, and seconds since midnight. Then the recorded time would appear, for example, as 16^h37^m52^S.71 which is read as "sixteen hours, thirty-seven minutes, and fifty-two point seven one seconds of time."

Units of time measure are sometimes used to designate the sizes of angles. The longitude of a point on the earths surface is often expressed in this manner. The relationship between the units of time measure and the units of angular measure in the sexagesimal system are as follows: