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Micah D. Cochran, MSCS

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combined_municipal_limits_geoprocessing

combined_municipal_limits_geoprocessing is a Python command line ETL script that combines City Limits from multiple GIS layers into one GIS layer using a unified schema for Limestone County (Alabama, USA). This takes five (5) files, with four (4) different schema and two (2) projections. The script merges the files are merged into one layer with one projection and one schema.

The script makes the process of updating the combined GIS layer fast, easy, and less error prone.

Source Data

GIS data from different sources has attribute tables that can vary widely:

Attribute table from Decatur City Limits.

Attribute table from Huntsville City Limits.

Attribute table from Madison City Limits.

Here are the five (5) different GIS layers as viewed GIS window:

The five (5) input GIS layers shown in QGIS software. Note: QGIS (software in the screenshot) has the ability to re-project data automatically. There are two CRS (projections) for State Plane Coordinates in Alabama: East and West.

Result Data

The resulting GIS data of the ETL script.

Single GIS layer of all of the municipal limits from QGIS map. The layers have had labels populated from the `NAME` column.

The resulting combined table (some columns have been omitted):

NAME	ProperName	MUNITYP	GNIS	LOCALFIPS	LASTUPDATE	ChangeDesc	MUNIAREA
Ardmore	City of Ardmore	City	1304866	01640	8/23/13	NULL	4.539
Athens	City of Athens	City	2403123	02956	2/23/21	Added Annex...	41.063
Decatur	City of Decatur	City	2404206	20104	6/10/19	NULL	60.786
Huntsville	City of Huntsville	City	2404746	37000	5/31/19	NULL	218.987
Madison	City of Madison	City	2404989	45784	9/2/16	NULL	30.478
Ardmore	Town of Ardmore	Town	2405163	02260	12/9/14	Verified limits...	2.050
Elkmont	Town of Elkmont	Town	2406437	23488	12/8/14	Mapped based...	1.725
Lester	Town of Lester	Town	2406007	42352	5/11/15	Verified limits...	1.326
Mooresville	Town of Mooresville	Town	2406188	51264	8/23/13	NULL	0.235

Screenshot of table in QGIS software.

Lessons Learned

I learned the hard way that Python virtual environment are a great way to keep an the API stable, so that the code will most likely still work later. Updates may have the unintended consequence of changing the API, for better or worse. Python virtual environments allow the developer to upgrade when the developer is ready to upgrade. Not when software in the rest of the environment has been upgraded.

ETL, GIS, Python, geoprocessing — May 1, 2023

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