Scrapes an Esri REST endpoint and writes a GeoJSON file.
If you just want to use the command line tool `esri2geojson`, the recommended way to install this package is to create a [virtual environment](http://docs.python-guide.org/en/latest/dev/virtualenvs/) and install it there. This method does not require that you `git clone` this repository and can get you up and running quickly:
pip install esridump
### Command line
This module will install a command line utility called `esri2geojson` that accepts an Esri REST layer endpoint URL and a filename to write the output GeoJSON to:
esri2geojson http://cookviewer1.cookcountyil.gov/ArcGIS/rest/services/cookVwrDynmc/MapServer/11 cookcounty.geojson
You can write to `stdout` by using the special output filename of `-` (a single dash character).
You can also pass in the `--jsonlines` option to write newline-separated (`\n`) lines of GeoJSON features, which you can then pipe into other applications.
### Python module
You can use this module in your code to get GeoJSON Feature-shaped Python `dicts` into your code:
from esridump.dumper import EsriDumper
d = EsriDumper('http://example.com/arcgis/rest/services/Layer/MapServer/1')
# Iterate over each feature
for feature in d:
d = EsriDumper('http://example.com/arcgis/rest/services/Layer/MapServer/2')
# Or get all features in one list
all_features = list(d)
The module will do its best to find the most efficient method of retrieving data from the Esri server, given [the capabilities of the server](http://resources.arcgis.com/en/help/arcgis-rest-api/index.html#/Query_Feature_Service_Layer/02r3000000r1000000/). There are several strategies we use to get the data, described here in most to least efficient order:
### `resultOffset` Pagination
In ArcGIS REST API version 10.3, Esri added support for pagination directly with the `resultOffset` and `resultRecordCount` parameters. Unfortunately, most servers don't support this feature because the backend SQL engine must also be configured to support it. So far, it seems that only the Esri-hosted layers support this feature reliably.
### `objectId` Field Chunking
In ArcGIS REST API version 10.0, Esri added support for the server to return an exhaustive list of object IDs for all features in a layer. Once this list of object IDs is retrieved, we break it into chunks of `maxRecordCount` queries using the `objectIds` parameter.
### `objectId` Statistics `where`-clauses
In ArcGIS REST API version 10.1, Esri added support for performing various statistical queries on the server without requiring the client to download the whole dataset. On servers that support this and don't respond to the `objectIds` queries, we will use a minimum and maximum statistics query to find the minimum and maximum values for the `objectId` column, then build chunks of `where`-clauses that narrow the range down to `objectId`s between two fenceposts.
### Geometry Quadtree Queries
When a server does not support any of these methods, we'll make recursive quad-tree queries using bounding envelopes. We start with a query for the layer's entire `extent`. If the server returns exactly the `maxRecordCount` number of features, we split that `extent` into 4 equal rectangles and query those. If those smaller queries return `maxRecordCount` features, we split the rectangle again and continue until the server returns something less than the `maxRecordCount`.
To suggest changes or improvements to this code, create a fork on Github and clone your repository locally:
git clone email@example.com:openaddresses/pyesridump.git # replace with your fork
Set up a [virtual environment](http://docs.python-guide.org/en/latest/dev/virtualenvs/) for development and install the dependencies for development:
pip install -e .
Your changes to the code will be reflected when you run the `esri2geojson` command from within the virtual environment. You can also run (and add) tests to check that your changes didn't break anything:
## See Also
TODO: Brief introduction on what you do with files - including link to relevant help section.