# Using TIN - interpolation plugin - to calculate volume

I am having trouble calculating the volume of my TIN.

I have attempted to create a TIN using the interpolation plugin in QGIS. Here are the steps that I followed.

1. I added a Zvariable row in the attribute table of the vector layer with the value of 500 (I am assuming this corresponds to 500 meters).
2. I used the interpolation plugin in QGIS to create a TIN layer. I created it following steps advised through the qgis website which recommended however that I use a cell size of 5000 for both both X and Y cells but this came up with an error message for me so I left it as it was set by default (please see the screenshot below for details of exactly what I entered).

3. After, I defined the coordinate system as WGS84, and the attached layer appeared (please see in the below attachment, the layer titled testTIN which is shown in black shading).

Do you believe that my TIN layer is correct and if so, how do I now go about calculating the volume of the area?

I have also attached for you a screenshot of the attribute table of the layer used in the interpolation plugin to create the TIN.

## 1 Answer

it is good that your detailed question has screenshots. I can see the project coorodinate reference system (CRS) is set to 4326, I presume the layer crs is this also.

This will mean that your units are degrees, which are quite extent compared to metres. A UTM CRS setting for your layer will work in degrees. For some tools in QGIS, they also interact with the CRS for the map canvas (project) and so it is good to be conservative and set that CRS the same.

The units in the dialog will be more obvious to you and easier to work with if you can have a CRS of UTM. So then a cell size 5000 in UTM would be 5 kilometres. In the 4326 CRS it would be a ginourmous (a valuable scientific unit) distance. On the same thread, your dialog would want a number of columns say 5000 with a cellsize of 1 - 10, if you use UTM.

• I am using 3 dimensional coordinates and over vast distances. 1. Is it possible to/ do I need to use a few UTM's at the same time? I have read that they are divided into 60 zones and that in order to get a specific area of interest (a point) you must first identify its exact UTM e.g (UTM 35S). I however need a large 3D area. 2. Your comment regarding the cell size. Do you mean that if I would set it to 1, the cell would be 1m long? And 400, the cell would be 400m long? In other words the cell size is always represented in meters? Sep 5 '14 at 7:36