A primer on map projections in Python

Ok, so after my post a little while ago about converting contours to shapefiles, I spent a day adapting the approach for use in a in a dash-plotly app… and in general it works, and took a ~800 MB file down to a ~14 MB shapefile. But the rendering of the shapefile was still very slooooooow, and due to limitations of how data callbacks update a figure’s data in Dash, I couldn’t just render the background map once. So, after some thinking, I realized that if I “simply” calculated my own map projections, I could use the standard x-y plotting routines (which includes contours, as well as the faster scattergl method for plotting a large number of points).

But then I realized that I only vaguely know what map projections are: I knew different projections vary in how they portray the 3D earth on a 2D surface, but when you start using all the mapping libraries out there, you run into a lot of jargon really fast. So here’s a super basic primer on calculation map projections in Python that explains some of that jargon!

Also, this is the first post I decided to write as a Python notebook. The notebook is in my github learning_shapefiles repo, but github has been having some issues displaying notebooks lately, so you can view the full notebook here.

PyProj

To start off, pyproj (link), is a python library based on the command-line library https://proj.org/ that provides a large number of algorithms related to map projections. So go install that (simple to do with pip or conda).

In order to use pyproj to project a lat/lon point onto a map projection, it’s helpful to know a few acronyms:

• CRS: coordinate reference system
• EPSG: a database of coordinate reference systems and transformations, different projections covering different regional or local (or global) domains are specified by the number following EPSG. For example, EPSG:4326 is the global reference ellipse used by modern day GPS.
• WGS84 : World Geodetic System, latest revision, a.k.a. WGS1984, same as EPSG:4326
• reference ellipse: an ellipse describing the surface of the Earth on which positions (like latitude and longitude) are defined. GPS points use a reference ellipse that approximates the Earth’s geoid (i.e., the gravitational equipotential surface that sea-level would follow if due to gravity alone, see wiki).

So now that’s out of the way…. to project a single lat/lon point with Pyproj, first import pyproj then initialize a projection (Proj) object:

from pyproj import Proj
p=Proj(proj='hammer',ellps='WGS84')

When initializing the Proj (projection) object, you give it the reference ellipse that your lat/lon is defined on, ellps=‘WGS84’, and the projection you want to transform to, in this case a hammer projection. Once you’ve initialized Proj, you can use it to move from lat/lon to cartesian x,y:

lon=-120.5 
lat=42.4
x,y=p(lon,lat)

This lon/lat point become (x,y)=(-9894366.0792,5203184.81636). These values in meters by default and are the cartesian x-y distance from the map center for your point.

The Proj object is flexible enough to take lists (and numpy arrays!), so you can project many points at once. It can also take any of the parameters defined for the PROJ projections as keyword arguments. For example, the Hammer projection, has an argument ‘+lon_0’ to set the 0-longitude for the view, and so in pyProj you can use ‘lon_0’ as a keyword argument when using the hammer projection:

p = Proj(proj='hammer',ellps='WGS84',lon_0=-50)

The rest of the notebook goes through how you could construct a grid of lat/lon lines and plot them for different projections. And at the end, I take a digital elevation model of the western US (which gives an elevation for lat/lon points), project the lat/lon points onto a hammer projection  to get points of (elevation,x,y) and contour it using the standard matplotlib.pyplot.contourf to get the following:

In my particular application, I’ll now be able to project my data to x,y and then use the cartesian contour functions of dash-plotly! The main drawback of this approach is having to write all the routines for handling rotations and plotting lines on maps… but ultimately I think it will work well with Dash, and it will be nice to be able use scattergl with my map point data.

 

Shapefiles in Python: converting contours to shapes

I’m in the process of finishing up a web-app that I’ve written using Dash/Plotly and as someone who is not an expert in front end web development, I’ve really enjoyed how easy it is to build an interactive app without getting bogged down by all the javascript/html/css (though knowing some definitely helps!). But recently I’ve run into an issue while figuring how to plot various datasets as a background contour on a number of interactive maps: while Dash/Plotly is great at plotting interactive points on a map, there’s not a simple way to contour a dataset and use it as the base layer (there is the new scattermapbox class, but there is not yet a simple contouring method for maps like, for example, contourf in the basemap library).

But Dash/Plotly is very good at chloropleth (colored shapes) plots, so I figured that if I could convert a matplotlib.pyplot.contourf plot into a shapefile of polygons, I could then load that shapefile in Dash/Plotly as needed and plot it as a chloropleth plot, which seems easier to me than rendering a bitmap image of the contour plot in the background (the only other option I can think of)… and so I spent the morning experimenting with how to do this. There’s a few scattered stackoverflow questions that related to this, but I had to pull together a bunch of pieces to get this working,  so I figured a short writeup here might be useful to others out there.

So with that said, here are they python libraries that you’ll need to run my test script:

• shapely (for polygons, used previously here, here and here)
• fiona (for writing a shapefile)
• descartes (for making some plots)
• matplotlib (for the initial contouring of the data)
• numpy (for making up some initial data and some array manipulation)

The full code is available in my learning_shapefiles repo and the new script is contourf_to_shp.py. And some useful stackoverflow answers that helped significantly: Converting Matplotlib contour objects to Shapely objects, matplotlib – extracting values from contour lines.

Creating data to contour 

To start off, I wanted to work with a small test data set that would include multiple domains at the same contour level, so I created a lat/lon grid and then superimposed two 2d gaussian curves with different amplitudes and decay rates and plotted those up:

from shapely import geometry
import matplotlib.pyplot as plt
from matplotlib import cm
import numpy as np
import fiona
import os,json
from descartes.patch import PolygonPatch

# create some test data with multiple peaks
lon = np.linspace(0,45,100)
lat = np.linspace(-20,32,90)
long,latg=np.meshgrid(lon,lat)
C1=np.sqrt((long-5.)**2+(latg-25)**2)/30.
C2=np.sqrt((long-30.)**2+(latg-1)**2)/10.
m = 30*np.exp(-C1**2)+20.*np.exp(-C2**2)

# make the contourf plot, storing the resulting ContourSet in cs
plt.figure(figsize=[10,5])
plt.subplot(1,2,1)
Nlevels=10
cs = plt.contourf(lon,lat,m,Nlevels,cmap='gist_heat')
plt.title('contourf figure with Nlevels='+str(Nlevels))

In my actual use-case I’ll be loading a 2D dataset, so most of this will be replaced. But the important bit is that I store the object that results from the contourf call, that cs object contains all the data on the contours.

Storing the contour results

The first step in saving the contoured data is to create a lookup table for the contour levels. The cs object stores each contour in cs.collections and the levels in cs.levels:

# create lookup table for levels
lvl_lookup = dict(zip(cs.collections, cs.levels))

Next, I loop over each contour of cs.collections, convert the contour’s shape into a shapely polygon and store it in a list of dictionaries that also saves that contour’s level value:

# loop over collections (and polygons in each collection), store in list for fiona
PolyList=[]
for col in cs.collections:
    z=lvl_lookup[col] # the value of this level
    for contour_path in col.get_paths():
        # create the polygon for this level
        for ncp,cp in enumerate(contour_path.to_polygons()):
            lons = cp[:,0]
            lats = cp[:,1]
            new_shape = geometry.Polygon([(i[0], i[1]) for i in zip(lons,lats)])
            if ncp == 0:                
                poly = new_shape # first shape
            else:
                poly = poly.difference(new_shape) # Remove the holes
            
            PolyList.append({'poly':poly,'props':{'z': z}})

The list PolyList now holds a shapely polygon as well as the z-value (or level) of that polygon.

Writing the data

In order to use these polygons elsewhere, the easiest thing to do is simply save them within a shapefile. I used the Fiona library for writing, which entails writing the geometry (the polygons) and the properties for each polygon that I stored earlier:

# define ESRI schema, write each polygon to the file
outfi=os.path.join(outname,'shaped_contour.shp')
schema = {'geometry': 'Polygon','properties': {'z': 'float'}}
with fiona.collection(outfi, "w", "ESRI Shapefile", schema) as output:
    for p in PolyList:
        output.write({'properties': p['props'],
            'geometry': geometry.mapping(p['poly'])})

In order to reconstruct the contour plot, I need to save the information about the different levels (to get the colormap correct). And while I could load the shapefile and loop over the shapes to calculate min/max values, it saves time to simply write that data to a different file. And rather than write it as metadata of some kind for the shapefile, I just drop it into a json file:

# save the levels and global min/max as a separate json for convenience
Lvls={'levels':cs.levels.tolist(),'min':m.min(),'max':m.max()}
with open(os.path.join(outname,'levels.json'), 'w') as fp:
    json.dump(Lvls, fp)

Plotting the polygons

From there, the shapefile and levels data can be re-loaded and plotted to get the following:

On the left is the original contour plot, on the write is the plot of Polygon patches colored using the same colormap. And it’s an almost perfect replica… I’m not sure if it’s my eyes or screen, but the lightest colors seem just a hair different between the two. But that difference is even less noticeable as I use more contours, e.g. 1000: 

The final bit of code is mostly self explanatory: looping over the shapes in the shapefile, then plotting colored polygon patches like in previous posts. The only tricky business is making sure the polygon facecolor matches the filled contour color. To do this, I load in the contour level json file that I saved off, and load the matplotlib colormap that I used in the initial call to contourf:

# read in levels, define colormap
with open(os.path.join(outname,'levels.json')) as jfile:
    Lvls=json.load(jfile)
levels=np.array(Lvls['levels'])
cmap=plt.cm.gist_heat
lv_range=[Lvls['min'],Lvls['max']]

And then for each polygon, I pull out the z-value that I saved and find the appropriate RGB value for that level with:

        lv=shp['properties']['z'] # this shape's level
        clr=cmap((lv - lv_range[0])/(lv_range[1]-lv_range[0]))

The bit of math with lv_range is ensure that the value is between 0 and 1. The clr variable is a tuple that can be given to the descartes PolygonPatch:

patch = PolygonPatch(poly, facecolor=clr, edgecolor=clr)

And that’s pretty much it. I still have some experimenting to do with how many contour levels I need for the datasets I’m working with, and I expect to run into some scaling issues (the full dataset fields I’m working with are 100s of MB), but at least I have a feasible approach!

Full code here.

Shapely Polygons: Expansion!

Today I found myself needing to scale a large number of geometric shapes corresponding to volcanic fields in the Western United States. Shapely (used in previous posts) makes this easy with the shapely.affinity.scale() method (an affine transformation preserves lines and points, and the scale() method is a simplified transformation without adding rotation or skew).

So I put together a test script using the scale() method (on my github here) that takes the scaling factor as input. Since I’ll be using this with geometric coordinates and will be testing sensitivity of measurements to different sized shapes, I added the option to scale the shape by a a sequences of distances (angular or linear). The code is fairly straight forward and I don’t have time for a detailed post, so for now, here’s a trippy plot showing a squiggly-circle shape scaled by different amounts:

To generate the above plot (assuming you’ve cloned my learning_shapefiles repo and are in the directory containing affine_expansion.py).:

python affine_expansion.py -f ../data/expansion_test_data.csv -exp 0.25,.5,.75,1
.25,1.5,1.75,2

 

merging shapes and plotting the physiographic boundary of the Colorado Plateau

Today I found myself needing to plot the physiographic boundary of the Colorado Plateau in Python. It’s been a while since I’ve touched on shapefiles (or anything on the blog) so I figured I’d write a quick blurb on reading and plotting this particular shapefile.

Data: shapefile of data from  Fenneman and Johnson 1946 [1] available at https://water.usgs.gov/GIS/dsdl/physio_shp.zip

Code to load & plot & write processed data: colorado_plateau.py

Python requirements: pyshp, shapely, matplotlib

What you’ll learn: reading shapefiles, merging polygon shapes in Python with shapely

The Data

The first challenge was finding the actual lat/lon coordinates defining the edge of the Colorado Plateau… it’s amazing how many papers in geology/geophysics plot the boundary but don’t actually reference where the heck they got their coordinates from. After much digging I FINALLY found a paper that actually cited their source: Hopper and Fischer 2018 [2] reference a 1946 publication by Fenneman and Johnson [1] titled “Physiographic divisions of the conterminous U. S.” and after a quick search I found the digitized data from that publication online at water.usgs.gov.

Here’s the summary page containing metadata: https://water.usgs.gov/GIS/metadata/usgswrd/XML/physio.xml

and a direct link to the zipped shapefile:  https://water.usgs.gov/GIS/dsdl/physio_shp.zip.

The dataset contains a large number of physiographic regions and the Colorado Plateau is subdivided into multiple regions, so the code below pulls out the regions within the Colorado Plateau and joins them into a single shape defining the full boundary. To run the code below, unpack physio_shp.zip wherever you downloaded it to and rename the folder to physio (to match expectations for the pyshp shapefile reader).

The Code

The full code is here.

The XML data for the shapefile defines a province code for different provinces, for which the Colorado Plateau sub-regions have a value of 21. So the code (1) reads the shapefiles, (2) finds the shapes with a province code of 21 and (3) combines them.

Step 1:  imports, reading arguments, reading the shapefile.

shapefile is the library for pyshp, otherwise pretty self explanatory:

import shapefile, os,sys
import matplotlib.pyplot as plt
from shapely.geometry import Polygon
from shapely.ops import cascaded_union

# read the arguments
fname=sys.argv[1] # path to physio.shp
outfolder=None
if len(sys.argv)>2:
    outfolder=sys.argv[2] # folder to store output

# read the shapefile
sf = shapefile.Reader(fname)

Step 2: Find the Colorado Plateau shapes.

The shapes are described in the records list of the shapefile object:

sf.records()

records() is a list of attributes for each shape and a single record looks like

[3.886, 9.904, 220, 15, 212, '21b', 'INTERMONTANE PLATEAUS', 'COLORADO PLATEAUS', 'UINTA BASIN', 21]

The final value is the province code — so we just need to save off the indeces for which that value is 21. It turns out the 3rd value in the record list is actually a cross-reference to a shape ID, but for some reason the indexing is offset by 2 when reading this shapefile with python. So the shape data for this shape would be accessed with:

sf.shapes()[218]

rather than 220. Not sure why it’s off by 2 (would expect it to be off by 1 due to python indexing), but in any case, my code simply records the list index as python sees it:

# find the record indeces for colorado plateau (province ID = 21)
i_rec=0
recs_to_plot=[]
for rec in sf.records():
    if rec[-1]==21:
        print(rec)
        print(i_rec)
        recs_to_plot.append(i_rec)
    i_rec=i_rec+1

# plot the individual records
plt.subplot(1,2,1)
for rec in recs_to_plot:
    pts=sf.shapes()[rec].points
    lons=[pt[0] for pt in pts]
    lats=[pt[1] for pt in pts]
    plt.plot(lons,lats,'.k')

As seen above — the coordinates for the shape boundaries for a record are in

sf.shapes()[rec].points

which is a list of longitude and latitude points (which the code unpacks for plotting). This section of code will generate the following outline of the Colorado Plateau regions:

Step 3: merging shapes

This is the fun bit! What we want is just the outer boundary of the union of all the shapes. The python library shapely lets us do this very easily by creating a list of shapely Polygon objects then combining them with the cascaded_union method:

# create a single shape for Colorado Plateau from union of sub-shapes
polies=[]
for rec in recs_to_plot:
    polies.append(Polygon(sf.shapes()[rec].points))
CP_bound=cascaded_union(polies)

# plot the exterior shape
lon,lat = CP_bound.exterior.xy
plt.subplot(1,2,2)
plt.plot(lon,lat,'.k')

and the resulting plot of just the exterior boundary:

Step 4: output the processed data 

The code also exports the lat/lon points defining that exterior boundary with:

# export the final shape as a CSV of boundary points
if outfolder is not None:
    f=open(os.path.join(outfolder,'ColoradoPlateauBoundary.csv'),'w')
    f.write("lon,lat\n")
    for i,j in zip(lon,lat):
        f.write(str(i)+","+str(j)+"\n")
    f.close()

I could have written some code to save the data in a shapefile format, but for such a small amount of data I find it easier to save a CSV and just create a Polygon from the list of points as I need it. I’m actually planning to create a Polygon that will be combined with GeoPandas to find sets of points falling within the plateau (GeoPandas lets you do database joins on geospatial data, it’s awesome!).

Running the Code

To run the code:

python colorado_plateau.py /path/to/physio/physio.shp /folder/for/output/

where the first argument is the path to the downloaded and unpacked shapefile and the second argument is the location to save the CSV file (this argument is optional — no data will be saved if not included).

References

[1] Fenneman, N. M., & Johnson, D. W. (1946). Physiographic
divisions of the conterminous U.S. Reston, VA: US Geological Survey,
Physiographic Committee Special Map. https://water.usgs.gov/GIS/metadata/usgswrd/XML/physio.xml

[2] Hopper, E., & Fischer, K. M. (2018), The changing face of the lithosphere-asthenosphere boundary: Imaging continental scale patterns in upper mantle structure across the contiguous U.S. with Sp converted waves. Geochemistry, Geophysics, Geosystems, 19 , 2 593 – 2 614 . https://doi.org/10. 1029/2018GC007476

Great Circle Paths

Been quite a while since any updates, but here’s a short one!

As a part of a contract I’m working on, I found myself having to plot the major arc of great circle paths on a map. But if you google “how to plot great circle path in [insert python library here]” all the solutions are for plotting minor arcs. Turns out in the end, there’s a really simple trick to plotting the major arc (and I felt pretty dumb when I realized it after wasting a ton of time), but I figured I’d write it up here in case it saves anyone else a bit of time. The short answer: given two points that you want the major arc for, just add the antipodes for each point to the list of points in your path.

First off, in case you need a review of great circles, here’s a globe:

Given two points on the surface of a sphere, there is a single circle that that contains both points (unless you’re at a pole, in which case there are infinite great circles). The short way round is the minor arc (red curve), the long way round is the major arc (green curve). And I needed to plot both of them.

The reason I got into plotting this in the first place is that in seismology, surface waves are described by major and minor arcs. When an earthquake generates seismic waves and is measured at a seismometer somewhere else, the raypath between the epicenter and seismic station falls on a great circle path. And surface waves are referred to in terms of the minor and major arcs: the R1 wave travels the minor arc and the R2 travels the major arc. These waves will actually keep going around the earth’s surface before dissipating: R3 is the R1 after it goes around again, R4 is the R2 after it goes around again, and on and on. So I needed to be able to plot all these.

Ok, so back to plotting…

I was using the plotly library in Python for this plot, so I’ll stick with that for examples here, but there should be similar functions in whatever mapping library you’re using. The full script is on my github page here.

So the important bit is just defining the list of latitude and longitude points. Here, the minor arc points are put into a dictionary:

paths={}
paths['minor_arc']={'lon':[ start_lon, end_lon ],
                    'lat':[start_lat,end_lat], 'clr':'red','dash':None}

When we give this to plotly, we’ll tell it to connect the two points, which will give us the shortest path between the two, the minor arc.

To plot the major arc, we just need to add some points between the start and end so that it takes the long way around. But how to choose the points? Well, turns out that there are tons of confusing pages out there on the trig used for calculating great circle paths, and I almost started to code up some of it… until I realized only needed a couple points. And the antipodes (the point that is exactly opposite a given point on the surface) are both real easy to calculate and  guaranteed to lie on the great circle path. Just add 180 to the longitude and flip the sign of the latitude:

ant1lon=start_lon+180
if ant1lon>360:
    ant1lon= ant1lon - 360
ant1lat=-start_lat

Same for the antipode of the second, end point. The >360 bit is just to make sure longitude remains between 0 and 360 degrees.

And now, we can put the antipodes in a list for the major arc:

paths['major_arc']={'lon':[ start_lon,ant2lon,ant1lon, end_lon ],
                    'lat':[start_lat,ant2lat,ant1lat,end_lat], 
                    'clr':'green','dash':None}

In the script, these paths are then added as a data dictionary used in creating the plotly figure:

DataDict=list()
for path in ['minor_arc','major_arc']:
    DataDict.append(
        dict(
            type = 'scattergeo',
            lon = paths[path]['lon'],
            lat = paths[path]['lat'],
            name= path,
            mode = 'lines',
            line = dict(
                width = 2.5,
                color = paths[path]['clr'],
                dash=paths[path]['dash'],
            ),
            opacity = 1.0,
        )
    )

figdata={}
figdata['data']=DataDict

The full script has a bit more where it actually plots the data (to give the image above), but plotly has some really nice tutorials for that already so I won’t bother explaining all that.

So that’s that! Hope it saves someone else some time.

 

A Python tool for inspecting shapefiles

In my recent coding exploits, I’ve downloaded lots of different shapefiles. Most shapefiles were accompanied by nice .xml documenation with information about the data and how its stored or labeled, but a few had hardly any information at all. I knew the general content based on the description from the website were I downloaded the shapefile, but I didn’t know what they had used for the record labels and I didn’t know what the record values were exactly. So the past couple days I sat down and wrote a bit of code to help in unraveling a myserious shapefile…

Check out (and/or download) the full Python source here: shapefile inspection!

The program is fairly straightforward. It traverses the records of a shapefile, recording the record label (or “field names” as I refer to them in the source) and information about each record. One of the program’s methods uses the Python XML API called ElementTree to  produce an xml file that you can load in a browser. Here’s a screen shot from using Firefox to view the xml file produced when running the program on the Open Street Map shapefile that I extracted via MapZen for my previous post.

In a browser, you can shrink or expand the xml attributes to get some basic information about each record: the name or label of the records, the data type and some sort of sample of the data. If the record data is an integer or float, then the sample will be the min/max values of the record while if it’s a string, it will either be a list of the unique strings in the records or just a sample of some of the strings. The OpenStreetMap shapefile contained some record values that were keywords, like the “highway” attribute in the screen shot above. While other records were strings with unique values for each shape, like the “name” attribute below:

In addition to generating an xml file, the program allows you to interactively explore a field.

When you run the program from command line (type in python inspect_shapefile.py in the src directory), it’ll ask for your input. It first asks if you want to give it a shapefile, here I said no and used the shapefile hardwired into __main__ of inspect_shapefile.py:

Do you want to enter the path to a shapefile? (Y/N) N
 
Using shapefile specified in __main__ :
directory: ../../learning_shapefiles/shapefiles/denver_maps/grouped_by_geometry_type/
filename: ex_QMDJXT8DzmqNh6eFiNkAuESyDNCX_osm_line

 Loading shapefile ...
... shapefile loaded! 

It then pulls out all the fields in the shapefile records, displays them and asks what you want to do. This is what it looks like using the OpenStreetMaps shapefile:

Shapefile has the following field names
['osm_id', 'access', 'aerialway', 'aeroway', 'amenity', 'area', 'barrier', 'bicycle', 
'brand', 'bridge', 'boundary', 'building', 'covered', 'culvert', 'cutting', 'disused', 
'embankment', 'foot', 'harbour', 'highway', 'historic', 'horse', 'junction', 'landuse', 
'layer', 'leisure', 'lock', 'man_made', 'military', 'motorcar', 'name', 'natural', 
'oneway', 'operator', 'population', 'power', 'place', 'railway', 'ref', 'religion', 
'route', 'service', 'shop', 'sport', 'surface', 'toll', 'tourism', 'tower:type', 
'tracktype', 'tunnel', 'water', 'waterway', 'wetland', 'width', 'wood', 'z_order', 
'way_area', 'tags'] 

Do you want to investigate single field (single)? Generate xml 
file (xml)? Or both (both)? single

Enter field name to investigate: landuse

So you can see all these different fields. I chose to look at a single field (“landuse”) and the program will then look at the “landuse” record value for each shape, record its data type and save new record values:

searching for non-empty entry for landuse ...
data type found: str
Finding unique record values for landuse
1 of 212550 shapes ( 0.0 % )
 new record value: 
93 of 212550 shapes ( 0.04 % )
 new record value: reservoir
6782 of 212550 shapes ( 3.19 % )
 new record value: residential
110432 of 212550 shapes ( 51.95 % )
 new record value: grass
111094 of 212550 shapes ( 52.26 % )
 new record value: construction
Completed field name inspection 

---------------------------------------
Shapefile has the following field names
['osm_id', 'access', 'aerialway', 'aeroway', 'amenity', 'area', 
'barrier', 'bicycle', 'brand', 'bridge', 'boundary', 'building', 
'covered', 'culvert', 'cutting', 'disused', 'embankment', 'foot', 
'harbour', 'highway', 'historic', 'horse', 'junction', 'landuse', 
'layer', 'leisure', 'lock', 'man_made', 'military', 'motorcar', 
'name', 'natural', 'oneway', 'operator', 'population', 'power', 
'place', 'railway', 'ref', 'religion', 'route', 'service', 'shop', 
'sport', 'surface', 'toll', 'tourism', 'tower:type', 'tracktype', 
'tunnel', 'water', 'waterway', 'wetland', 'width', 'wood', 'z_order', 
'way_area', 'tags']

The field name landuse is str
and has 5 unique values
Display Values? (Y/N) Y
 possible values:
['', 'reservoir', 'residential', 'grass', 'construction']

As you can see from the output, there were 4 keywords (reservoir, residential, grass and construction) used to describe the ‘landuse’ field. So I could now write some code to go into a shapefile and extract only the shapes that have a ‘residential’ value for ‘landuse.’ But I couldn’t do that until I (1) knew that the landuse field existed and (2) knew the different definitions for landuse type.

So there it is! That’s the program. Hopefully all the shapefiles you ever download will be well-documented. But if you find one that’s not and you really need to figure it out, this little tool might help!

Some code notes and tips

The xml file that I create didn’t follow any particular standard or convention, just what I thought might be useful. Perhaps that could be improved?

REMEMBER THAT IN PYTHON, YOU NEED TO EXPLICITLY COPY LISTS! I stupidly forgot that when you make a list

list_a = list()
list_a.append('blah')
list_a.append('d')

And then want to make a copy of the list, if you do this:

list_b = list_a

Then any changes to list_b will change list_a. But if you do

list_b = list_a[:]

You’ll get a new copy that won’t reference back to list_a. This is probably one of the things that I forget most frequently with Python lists. Palm-smack-to-forehead. 

The XML API ElementTree was pretty great to work with. You can very easily define a hierarchy that will produce a nice xml tree (see this example). I did, however, have some trouble parsing the direct output from the type() function. When you calculate a type,

type(0.01)

you get this:

<type 'float'>

When I gave it directly to ElementTree (imported as ET here), like this:

ET.SubElement(attr, "attrtype",name="data type").text = type(0.01)

I would get some errors because of the quotation marks enclosed. To get around this, I converted the type output to a string, split it up by the quotes and took the index that would just be the type (int, str, or float):

ET.SubElement(attr, "attrtype",name="data type").text = str(type(0.01)).split("'")[1]