Portable surveying instrument that consists of a drawing board and a ruler (alidade) mounted on a tripod and used to sight and map topographic details and to plot survey lines directly form field observations. This piece of equipment is much used in earlier surveying and map-making. One end of the alidade is held on the point on the map representing the point of operation, and the other is directed at a marker on the point to be plotted. This gives the angle from the point of operation, and distance can be plotted directly along the ruler after scaling down from the original measurement. The technique has been replaced mainly by photogrammetry.