One thought on “Q: What is the difference between elevation and RL?

  1. To understand the difference between “Reduced Level” ( RL) and Elevation, in the context of engieering projects/mining projects we need to discuss about a time when modern satellite based digital survey instruments were not available. The surveyors still prepared very accurate maps, even inside deep forests without using the latitudes and longitudes. For this they just created a local grid !! For example, a point on the surface in the southwestern corner of the area to be mapped is arbitrariy fixed as the ‘origin’ point and is assigned a ‘latitude’ of say 20,000m and ‘departure’ of 20,000m, as X and Y coordinates respectively. This is almost similar to the origin point of a graph paper which reads 0 and 0 along X and Y. Now to fix the elevation variation (Z) , they assigned an arbitrary value to the same origin point, say, 100m. The X,Y, Z co-ordinates of the origin point are now 20,000m, 20,000m and 100m respectively. This point was used as a local datum. All features on the ground were plotted with respect to their distance from the origin point along the X and Y axes in the local grid. A bore hole collar located at the intersection of X 20,350.55m and Y 25,550.65m could be accurately plotted. The elevation of points were measured with respect to the same origin point as “Reduced Levels” using an instrument known as Dumpy level which measures the vertical level difference between any two points. In this way the surveyors were able to generate X, Y. and Z values of any point on the ground ( I am not going in to the details of use of Theodelite, triangulation, Telescopic aledaide etc). Such maps were very accurate within the survey area. With the advent of technology the distinction between RL and Elevation is lost although the word RL is still in use to describe the elevation of bore holes, tunnels, benchmarks etc. Surveyors presently use the UTM coordinates for X and Y, determined using a DGPS and elevation (Z) is tied to a global geodetic datum such as WGS 84.


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