Purchase of this book includes free trial access to www.million-books.com where you can read more than a million books for free. This is an OCR edition with typos. Excerpt from book: CHAPTER IV. LEVELLING AND TOPOGRAPHY. Levelling. Levelling is theMorePurchase of this book includes free trial access to www.million-books.com where you can read more than a million books for free. This is an OCR edition with typos. Excerpt from book: CHAPTER IV. LEVELLING AND TOPOGRAPHY. Levelling. Levelling is the art of finding how much one point is higher or lower than another.
A Level Line is one which is perpendicular to the direction of gravity. A Levelling Instrument is any instrument by which a level line can be accurately determined. A Datum Plane, or Datum, is the initial plane or point in the plane from which all heights or elevations are computed. The Elevation of a point is its height when referred to datum, or its vertical distance above or below datum. A Levelling-rod is a graduated staff for measuring the distance from the line indicated by the instrument to the point whose elevation is desired.
A Target Rod has a sliding disc which is moved by the rodman to the position indicated by the man at the instrument. The rodman is expected to call off the reading of the figures as indicated by the position of the disc. A Speaking- rod is one graduated with such distinctness that it can be read by the instrument man with precision. The speaking rod is preferable for use in all drainage work. Bench-marks are permanent objects whose elevations are determined and recorded for future reference. Levelling Instruments.
The engineers spirit-level in some of its more or less expensive forms is the most accurate instrument and the one with which the most rapid work can be done. Telescope-levels are sufficiently low in price, so that it is not wise to use the poor and cheap kind of levels which are recommended by many as sufficiently accurate for drainage purposes, especially where the work to be done is of any magnitude.
The fact that most of our drainage work requires the utmost accuracy attainable is not appreciated by many. The care and adjustment of the instrument should be learned and can us...