Vertical lengths of pipe are calculated using center line elevations of pipe lines, while horizontal lengths are calculated using north-south and east-west coordinates. Pipe lengths are determined through calculations using coordinates and elevations. Because isos are not drawn to scale, accurate dimensions are required to specify exact lengths of piping runs. The Iso, as isometric is commonly referred, is oriented on the grid relative to the north arrow found on plan drawings. The symbols that represent fittings, Valves and flanges are modified to adapt to the isometric grid. Isometrics are usually drawn from information found on a plan and sectional elevation views.

Unlike orthographic drawings, piping isometric drawings allow the pipe line to be drawn in a manner by which the length, width and depth are shown in a single view. Sectional views are required for clarity of pipe routing More number of drawings needed in orthographic views than ISO to represent the same piping system. In a orthographic view it is not a problem if the pipe runs in one plane, but when a pipe runs in two or three planes (north to south, then down and then to the west, etc.) an orthographic view can be unclear. Simplicity (Only one pipe line is drawn in one A3 size paper). Also pipe fabricator can pick up material required for fabrication of pipe line from stores, with the help of Bill of Material given in isometric drawing. A pipe fabricator can read the drawing and understand pipe routine easily. An isometric drawing is generally on a A3 size paper. To aid pipe fabricator on site, isometric drawings are prepared. But 3D scale model can not be put on a paper, and 3D scale model does not indicate dimensions.

A pipe fabricator can understand GA drawing better only with the help of 3D scale model. A0 or A1 size drawing is to big to handle. A GA drawing is generally on A0 or A1 size paper and contains a lot of lines, and may not be easy to understand pipe routine.

2D General Arrangement (GA) drawings are used for fabrication on site. This is then converted into a 3D scale model, to check interference of pipe lines. In this all pipelines are drawn at zero z co-ordinate. Why Isometric Drawings are required?: Generally in the development of a project, a conceptual 2D drawing is first created.

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