PyShp Attribute Types and Point files

A quick note on attribute types when using pyshp, these have always seemed really clunky to me with a lot of trial and error needed to get things working, but I came across the xbase documentation which is the format used to store the attributes in shapefiles and this document is a treasure trove of information.

The following information is of particular relevance in terms of storing geodata data as attributes, which I could never get right before:

• C is ASCII characters
• N is a double precision integer limited to around 18 characters in length
• D is for dates in the YYYYMMDD format, with no spaces or hyphens between the sections
• F is for floating point numbers with the same length limits as N
• L is for logical data which is stored in the shapefile's attribute table as a short integer as a 1 (true) or a 0 (false). The values it can receive are 1, 0, y, n, Y, N, T, F or the python builtins True and False

Each of these datatypes is stored in an attribute created by pyshp using the code:

Where the two numbers are the length and the precision of the numerical value being stored. These are not needed for integers and non numeric datatypes, but a length of 1 is needed for logical data.

I have written a small code sample which creates data from lists to show the different datatypes being used correctly in pyshp to create points:

Hopefully I will have time soon to write up examples for polygons and lines and more complex use cases of pyshp.

Correcting GPS Coordinates

Today I was given an excel sheet containing an X and Y column of GPS coordinates that someone in another department had attempted to display in ArcMap. They were successful in that they got some points to display, but these points were located somewhere in the Atlantic, I was asked to 'fix' it.

In these situations more information is always useful, but none was forthcoming, so I made an educated guess. Often when GPS units capture data in British National Grid, they miss out the first number of each X and Y value as these correspond to the map sheet identifying letter and rarely change for the average user. These letters have numerical equivalents, so I tacked on the correct numbers to each coordinate pair and displayed them in Arc. Lo and behold, they were now in the right place.

I decided to knock together a quick Python script to automate this process, which gave me the opportunity to learn how to interface with excel sheets directly (I would usually export to *.csv or *.dbf) using the xlrd module and refresh my memory about the idiosyncrasies of the wonderful pyshp module. Below is the code I wrote in an hour or so. Originally I was going to write a small interface for it and pass it on to the department, but that is a task for another day. It's a bit ugly, but it does what is needed.