Hello. I’m using NHGIS Census Tract (and Block Group) GIS files for 1990, 2000, and 2010. In the 2000 and 2010 files, there are ALAND and AWATER variables to distinguish water from land area within a given CT or BG. In the 1990 files I have only seen SHAPE_AREA variables. I’m not sure if SHAPE_AREA is the total area for water+land, or if there are separate water and land area variables available for 1990.
Basically, I need to have a variable for area across all years that uses a consistent method so it is comparable, and I would prefer to use the land area variable if available across those 3 years. Can anyone provide clarity for this for 1990 or know where I can find documentation about this? Thank you!
The SHAPE_AREA field gives the total area in square meters for each polygon in the NHGIS shapefile, so it generally excludes offshore waters (coastal and Great Lakes areas, which NHGIS shapefiles clip out) and includes any other internal water areas.
The Census’s land and water area figures are also generally available in the Summary Files that are the source for NHGIS data tables. For basic space-saving reasons, however, NHGIS has generally omitted these fields from output extract files. That said, we’ve long been aware that they’d be useful for many users, and we plan to start including them in extract table files soon.
To help you right away, I just pulled the AREALAND and AREAWATER fields from our internal NHGIS data file for census tracts in 1990 STF1, and I put them in a CSV file at https://assets.nhgis.org/1990_STF1_tract_areas.csv. It contains just three fields–GISJOIN, AREALAND, and AREAWATER–with a header row. I expect the areas are given in square meters, rounded to a whole number.
Feel free to reply if you have any more questions about this!
Hi Jonathan. Thank you so much for this clarification and for providing the download. This is extremely helpful. Excuse my ignorance about this data, but is there a way for me to access the internal NHGIS data file to pull data the way you did? For example, I’ll also need this for 1990 Block Groups, and I couldn’t find a way to get it within 1990 STF1 through the regular NHGIS download interface (none of the STF1 tables I found seemed relevant, but I think maybe I just don’t know which table it should be in?). Sorry if this is an obvious question! I think I’ve found a way to access it outside of NHGIS but I thought it would be simplest and cleanest to get it all in one place. Thank you!
No, there’s no way for external users to access the internal files–other than for us staffers to share on request, as I’m doing now! I’ve put the areas for block groups up in another CSV at https://assets.nhgis.org/1990_STF1_bg_areas.csv.
As I said, we do plan to start including the area fields in output extract files (the files that external users get), but we haven’t yet, so for now, I think this is the simplest option… assuming we’re not inundated with more requests like this! (That hasn’t been a problem yet 
)
Okay, got it. Thank you so much for your help on this!
On October 28, 2022, we released wide-ranging updates to the context fields for NHGIS data tables, including the addition of land areas in 1990, 2000 & 2010.