Eliminate the Need of Planimeter! | How to measure area of a toposheet

A planimeter, also known as a platometer, is a measuring instrument used to determine the area of an arbitrary two-dimensional shape.


Using a planimeter could be extremely tedious could associate your work to a lot of errors, so under such circumstances AutoCAD or any other CAD software could be of great aid. All you need is a scanner to scan the plan or geometry and convert it into its soft copy in PDF (portable document format). Once this task is accomplished calculating and manipulating area and geometry is an extremely easy task.



In the Current Context I shall be demonstrating this technique with an assignment we executed in our Engineering!



As under the subject of HWRE we had to estimate precipitation of a catchment from a toposheet

Following is the an example of a toposheet:

Example of Toposheet

What we did is traced out the catchment area from the toposheet and scanned it to convert it into its PDF soft copy.

Original Scanned Copy Of Tracing Paper

Once You have the softcopy of the tracing paper, import it into AutoCAD using the PDF underlay feature of AutoCAD

The following Video Explains the PDF underlay feature in 2 minutes...

Once you've imported the files in AutoCAD just take the polyline command and start tracing the geometry of the PDF underlay.

Traced By Using Poly Line

After tracing the Geometry you need to scale it.

Scaling the Geometry is a bit of a task and requires some calculations.

• Take a dimension you know for instance in my case the distance between P3 and P4 was 6.5cm so first scale the geometry in such a way that you get the same dimension when measured by dimension command (DAL)
• Once the above step is completed you need to move scale the geometry to the actual scale it was designed for in my case the scale on the plan was 1:50000 so I scaled the figure by a multiplication factor of 50000
• Now your figure is to the scale but it still needs a bit of modification.
• The distances you measure won't amount to the realistic value so you need to again scale the geometry by using the scale command and a scale factor of 1/1000.
• Finally your figure is completely ready for further execution.

                                                      The following Video Explains the Scale command in AutoCAD

Now you just need to use a few AutoCAD commands and mark the polygons and Isohyets using the line, polyline and other drafting commands.

 

Now by using the boundary (bo) command create boundaries of the area of influence of the Rainguage stations and after that just select the boundary line and then type li (list command). This command will display the area and perimeter enclosed by the polyline.

For doing this I recommend working on Layers as it will enhance the effect and ease in drafting.

                                                   The following Video Explains how to calculate area  in AutoCAD

Finally you can compute the areas observed along the rainfall data and perform the precipitation analysis as follows

Following is an example of how I calculated the precipitation using SpreadSheets

Hope this was certainly useful...
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