Create Thiessen Polygons using ArcMap and Calculating Thiessen Weights

welcome to another rjs tutorial in today's tutorial I will show you how to construct these--and polygons using ArcGIS so for today's tutorial just to give you a bit of a perspective I have chosen a catchment in South Africa and my objective today is let me just go ahead and turn off this base map you can see there is a layer called rainfall stations over here so I'm just going to turn that layer on and my today my objective is actually to create sort of calculate the contribution which I can get from each of these different rainfall stations to my catchment which is which is represent by B which is represented by this purple color boundary now even if you look at the the proper definition of Thiessen polygons it says that this in polygons are generated from a set of sample points such that each polygon defines an area of influence around its sample point so that's exactly what we are going to do today in order to do that first you need to make sure that you have already imported you your points layer into your arcmap interface and and the creation of the Thiessen polygons is actually going to be purely based on the on the on the spatial distribution of your points so let's go ahead and go to your search panel and you can search Fusion polygons and over here you can see the under the analysis option there is one tool called create this in polygons alright now as your input features as I told you your Thiessen polygons are going to get created based on the spatial distribution of your points so you select your rainfall stations as your input features and the output feature class I'm just going to leave it as it is all right now you can see that we successfully created we were able to successfully create the Thiessen polygons if I just move in my catchment above this Thiessen polygon layer and as you can see actually each of these polygons specify its area of influence in this case the area of influence of each of the rainfall stations so that was quite easy and the next step what I'm going to do is actually I'm going to clip this TCN polygons layer from my catchment boundary because I'm not really interested in the areas beyond my catchment boundary so I'm just going to perform a simple clip operation go to clip analysis tool and my input feature is now going to be the newly created TC and polygon layer and I'm going to clip it with the boundary of my catchment click OK alright now if I turn off these two layers you can clearly see actually what is the area of influence to my catchment from each of the rainfall stations if I just go ahead and change the symbology a little bit maybe select a different color scheme like this all right now that's a much better representation of the tip of TC and polygons now the next thing what you would normally want to do is actually calculate the TC and Bates that means if you take this small part of the catchment what is the weight edge that we can actually assign for this part of the the catchment which is actually going to get multiplied by the the rainfall contribution this particular station and if you look at one station inside our catchment for example this station after I know the weight edge of this particular green color polygon I'm just going to multiply that red edge by the the rainfall contribution from this particular station and in a similar manner I'm just going to do that for all of these individual polygon parts and after that if I sum up everything all together with weights assigned to each polygons I'm sort of getting one rainfall value or it could be it could be either one rainfall value or it could be even a time series of rainfall which takes into account the weights from the Thiessen polygons so that's what my objective is from this tutorial so in order to do that first you need to calculate the areas of each of these polygons now if you want to calculate the area's if I check over here when I move my mouse pointer you can see that my units are in decimal degrees now if I go ahead and check in one of my layers go to properties and insource you can see my current coordinate system is actually a geographic coordinate system which is WGS 1984 but I would like all my units to be in meters so in order to do that what I'm going to do is I'm just going to actually reap reject everything that I have in my current data frame into into a projected coordinate system only then I'm going to calculate the areas now as I told you this is since this catchment belongs to South Africa I'm going to use the the South African transverse Mercator projection which is known as hardy base talk 94 so there are actually multiple ways of reproject in your layers so I'm just going to use one of the simplest methods that is actually to change the projection system of the main data frame just going to search it and under projected coordinate systems National Grid's South Africa she's going to select this 94 L or 15 click apply yes all right now in order to in order to reap reject your TC n polygons layer what I'm going to do is I'm just going to simply go to data and export data and when I'm exporting I'm just going to select my coordinate system to be the coordinate system of the data frame which we just change into a projected coordinate system so I'm just going to specify my saving location I'm going to name this as T P stands for TC in polygons save and press ok so I'm just going to remove this one in a similar manner I'm also going to reproject my catchment the river and the rainfall stations as well export data select the data frame and this one I'm going to name it as catchment actually I wouldn't really need the river to be projected but in any case let's reproject it as well to the data frame is going to be and finally the rainfall stations [Music] all right now if you just go ahead and go to one of the properties of one of these layers now you can see that the coordinate system has been changed into a projected coordinate system which lets us do our work in in in meters instead of decimal degrees so I'm just going to go to the data frame again go to properties and over here I'm just going to change my units into into meters all right now you can see that even over here our units got changed into meters and then the next thing that I'm going to do is I'm going to calculate the weights for each of these polygons so let me just go ahead and change the symbology again just to just for the sake of visualizing it all right so what I'm going to do now is I'm just going to go ahead and go to the attributes table of this these--and polygon layer and now you can see that we have ten polygons the field idea starts from zero and ending in nine we have ten different polygons and we also have one column called shape area in any case I'm just going to create a new column called area keep this one in double and over here by right-clicking I'm just going to go to calculate geometry I'm going to calculate all the areas in let's say square kilometres all right now you can see it shows us the individual areas of each of these polygon parts but now I'm interested in actually knowing what is the area where edge of each of these polygons so that I think you might be able to guess by now is equal to the area of each of these polygons divided by the total area so first I need to know what is the total area of all of these polygons added together that means basically my catchment area so in order to know that you can actually select here and go to statistics and here under the field area you will be able to actually see this descriptive statistics such as the mean standard deviation the minimum and maximum but what I'm interested right now is this value which is the sum so I'm just going to go ahead and copy that and I'm going to create another column called weight which is also of the format double and now I'm going to go to the field calculator change the path to Python and I'm specifying the weight of each of these polygons to be the area the column which I just created divided by the total area of the catchment all right now you can click OK all right now you can see that the weights got calculated without any issue just going to go ahead to the properties and over here you can change actually how many decimal points you would like to like in the attributes table to appear just going to keep it simple just two decimal points all right and something is that we could do is to actually display each of those weights go to label features in this layer and here select the newly created column called weights with the font size you can play around and when you click OK you can actually now see all the weights which corresponds to how much of a contribution each of these rainfall stations would have for this particular catchment so if you were to calculate let's say a generalized point rainfall you have to multiply each of the rainfall values by this weight and when you sum everything up together you will actually get the sort of the representative rainfall for this particular catchment from all of the the contributing stations so that's about it for this tutorial I hope you enjoyed if you have any further questions do come in them down below and as you might know already we produce many interesting CAS tutorials almost every week so if you would like to stay updated on the nuke on cool new techniques on the world of GIS you actually can subscribe to this channel as well so I'll see you in the next one thank you

Channel: GeoDelta Labs

Views: 33,286

Keywords: Thiessen, Polygons, Weights, ArcMap, ArcGIS, QGIS

Id: BPjScQ-fovU

Length: 14min 43sec (883 seconds)

Published: Fri Mar 22 2019