{"id":1261,"date":"2016-03-29T21:37:17","date_gmt":"2016-03-29T21:37:17","guid":{"rendered":"http:\/\/blogs.lincoln.ac.nz\/gis\/?p=1261"},"modified":"2023-05-07T04:05:58","modified_gmt":"2023-05-07T04:05:58","slug":"python-scripting-with-the-field-calculator","status":"publish","type":"post","link":"https:\/\/blogs.lincoln.ac.nz\/gis\/python-scripting-with-the-field-calculator\/","title":{"rendered":"Python Scripting with the Field Calculator"},"content":{"rendered":"

This post details how you can use some Python scripting to do if\/else statements in the Field Calculator.<\/em><\/p>\n

If the title of this post and the intro haven’t put you to sleep already, read on! \u00a0Perhaps they’ve even piqued your interest! \u00a0(There are support groups available for this condition.)<\/em><\/p>\n

I was recently contacted by a keen GISer (who shall remain nameless) with a problem they were having. \u00a0They were wanting to do some work around the Land Cover Database (LCDB) – a vector polygon layer, with up to 33 classes of land cover across the whole country. \u00a0To cover the whole country it uses a set of 479,353\u00a0polygons! \u00a0Previous versions were released for 1996, 2001, 2008 and this latest version has 2012 data, thus providing views\u00a0of New Zealand’s landcover across those years, like snapshots in a photo album. \u00a0The lastest version (4.1)<\/a> includes data from the three previous versions so, in theory, one could track changes in land cover from 1996\u00a0\u00a0to 2012. \u00a0Here’s a shot of a portion of the table, focusing on the class names:<\/p>\n

\"LCDBTable\"<\/a><\/p>\n

So, this person was interested in looking at landcover\u00a0changes at particular study sites from one year to the next, with an eye towards quantifying changes based on % of total area in the study areas. \u00a0In the table above, most of these polygons haven’t changed across the years, but several have. \u00a0Some have gone from High Producing Exotic Grassland to Exotic Forest – and they wanted to quantify changes like this. \u00a0But how to do so?<\/p>\n

There’s a Feature Compare<\/a> tool (sort of like Compare in Word) that lets you look at differences between two layers, but we were\u00a0looking for changes within<\/em> the table of a single layer; a different proposition. \u00a0My first thought was \u00a0to give yourself a new attribute that would signal if there had been changes from one year to another (a slightly simplified version of what they were after), which got me thinking about “if\/then” conditional statements. \u00a0And this takes us down the road of some simple scripting – abandon hope, ye who enter here…<\/p>\n

\"Abandon<\/a>
Source: http:\/\/viralyster.com\/a-helpful-illustrated-guide-to-dantes-inferno\/<\/figcaption><\/figure>\n

Python is a useful scripting language and is the preferred one for ArcGIS. \u00a0We’ve covered some\u00a0scripting previously<\/a>, and here I was interested in using some very simple Python to get this done within the table. \u00a0After creating a new integer field in the table to store an integer (let’s call it Comparison), the basic idea was to:<\/p>\n

    \n
  1. compare the landcover class names between two different years, and;\n