Module 8 - Working with Geometries

This week’s lab dealt with working with geometries in Python, and it was a challenging one for me, partially because working with arrays and nested loops are not my strong suit and partly due to a pretty intense work schedule this week. In this lab, we were working with the “rivers.shp” file, using loops to iterate through the cursor, and we wrote the data to a .txt file.

After importing the modules and setting the workspace, I created a search cursor for the shapefile I was working with. This called the OID field, the SHAPE geometry object, and the NAME field. I created a .txt file using the write mode, following examples in the text and in our exercise. To write the data to a text file, I needed to create a couple of “for” loops. The first iterates through each row in the shapefile’s attribute table. I created a variable called vertexid within the loop set to 0, so that it resets for each new feature. Within that loop, I created a second loop using the .getPart() method, which iterates through each point in the array of the row. The .getPart() method allows me to access the points in each array. If I did not use this method, another “for” loop would be necessary. I added “1” to the vertexid variable in this loop to keep track of the number of points in each feature.

Here’s where it got tricky for me. I needed to use the write() method to write to the .txt file. I wanted to add the Feature/row ID, the vertex ID, the X and Y coordinates, and the name of the river feature, using “\n” to ensure each vertex is on its own line. I ran into a wall here with syntax errors and exceptions at first. I was pretty sure the loops were correct (at least mostly) because I immediately was able to print correct values for the X and Y coordinates. I ran into a syntax error which gave no input into where or what the error was, so I started deleting code (remembering later I could have commented it out) and attempted to run the program. The issue turned out to be a print statement I had created underneath the write() statement, which wasn’t necessary, so I just left it out. The other error I had was not using arrays properly. I had the output in the .txt file correct, but the name and feature were printing “NAME” and “FEATURE”. Once I changed this to row[2] in the write() statement, the program wrote to the .txt file correctly. The output is shown below.  The first column shows the Feature/row ID, the second column shows the vertex ID for that feature (notice how it resets for each feature), the 3^rd and 4^th columns are the X and Y coordinates of the vertex, and the 5^th and 6^th columns are the feature name and type.

This was a really good lab, as I learned about working with geometries and got more experience working with arrays. Both the instructor and the other students on the discussion boards were a big help with steering me in the right direction with my code. I wish I had had more time to really work with it, as opposed to an hour or two at a time, but even so, I learned a lot and I think it’s less confusing and less intimidating to work with than with FORTRAN or even IDL code.