Since October 2019, DataWorks has been reaching out to Durham community members and organizations about improving the environmental data in the Durham Neighborhood Compass. We started with a presentation and discussion at the Environmental Affairs Board and followed up with a workshop of diverse stakeholders at the WG Pearson Center in February.
Each of these conversations, and time spent with the People’s Alliance Environmental and Climate Justice Action Team, have yielded a number of potential improvements and additional environmental data to support community work in Durham. We’re planning to continue updating and refining environmental metrics throughout 2020. This post is an update on where we have gotten with key land cover data points: tree canopy and impervious surface.
Beginning June 26 the Compass will publish new, revised, estimates for the percent of Durham County blockgroups covered by tree canopy and impervious surfaces, for 2011 and 2016. These variables had already been present in the Compass for parts of Durham, drawn from the EPA EnviroAtlas project. But the data were growing outdated, derived from 2009-2011 aerial imagery. Furthermore, those data were only available for the urban area of Durham, excluding rural areas of the county and environmentally sensitive areas near Falls Lake. And they were not being updated regularly.
Moving to NLCD
The National Land Cover Database (NLCD) is updated every 5 years with 30-meter resolution land cover classifications made available for the continental United States. Impervious surface and tree canopy variables drawn from it represent the percent of total land within each block group that is impervious or covered by tree canopy.
Using NLCD-derived variables will come with some essential improvements:
- Extending coverage to all of Durham County and for both Census Tracts and Blockgroups;
- Provide consistent data and analysis of the data for each 5-year update;
- Allow for additional regional trend assessments or cross-city comparison of these key environmental variables.
The NLCD estimates are more reliable for comparisons across time and with different parts of the United States, but on a blockgroup-by-blockgroup level they sometimes differ significantly from the EPA EnviroAtlas estimates which we previously used in the Compass. We’re seeing NLCD values at the blockgroup level which differ from the EnviroAtlas values by an absolute difference of -5.8% to 33.4% for tree coverage and 0.8% to 16.9% for impervious surface. On average the NLCD values for both tree coverage and impervious surface are slightly larger than the EnviroAtlas values. If you’re using these Compass variables in your research or in a project, you can review the specific changes in detail in this spreadsheet or contact us to discuss further.
What You’ll See in the Compass (And What To Do With It)
Going forward the Compass will include all of the available time series from NLCD, starting with 2001 and going through 2016 (for impervious surfaces) and 2011 to 2016 for tree canopy. These will help describe the changing intensity of heat islands and stormwater flooding, as well as where new development is causing potentially harmful impacts to drinking water, natural habitats and neighborhood quality of life.