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Stretches of secondary fencing are topped with spirals of concertina wire alo...

Credit:Brandon Quester / inewsource

Above:Stretches of secondary fencing are topped with spirals of concertina wire along the U.S.-Mexico border between the San Ysidro and Otay Mesa ports of entry, in San Diego on Aug. 16, 2017. Border Patrol agents use the frontage road between this and primary fencing to patrol for immigrants attempting to enter the U.S. illegally.

How We Crunched The Numbers

About this project

“America’s Wall” is a KPBS and inewsource investigation about the existing barriers along the U.S.-Mexico border ahead of President Trump's promised wall. Using previously undisclosed government data, we reveal the impact of construction over time. Learn more.

America’s Wall is the culmination of months of reporting and data analysis to better understand the evolution of the U.S.-Mexico border wall and its impact on illegal immigration.

Other news organizations have reported extensively on the length — and gaps — of the border wall, including many efforts to map where today’s fencing exists.

KPBS and inewsource were interested in the specific fencing types and locations so we could build a precise map of the modern wall. To do that we filed a Freedom of Information Act request to U.S. Customs and Border Protection asking for detailed mapping data showing the location and makeup of every piece of the wall.

In response to our request, CBP provided two digital map files detailing more than 3,600 individual sections of the border wall. These records formed the foundation of our reporting.

As with every major data project, for complete transparency and disclosure, we’re sharing our process and data methodology.

For each section of wall, the files included information such as type of fencing, year construction was completed, and the state and Border Patrol sector in which it was built. The oldest section of wall in these records dates from 1962. The newest section is from May 2015.

CBP grouped fencing under two umbrella types based on what the barrier was intended to stop: pedestrians or vehicles. Those umbrella types were broken down further into more specific subtypes: Pedestrian fencing consisted of primary, secondary and tertiary sections; vehicle fencing was divided into permanent and temporary structures. You can read those definitions here.

Each piece of fencing included a “date_in” attribute, which details when construction of that piece of fencing was completed. But in our vetting process of the data, we came across sections that either did not have a specific year attached or which had an estimate for the year of completion.

These undated sections account for 0.18 miles of total fencing, or 0.02 percent of the total wall. We included them in the total fencing calculations used throughout this project and referenced in our map, but the actual polygons, or shapes detailing pieces of fencing, were not plotted on our interactive map.

We handled on a case-by-case basis the fencing that had an estimate for when it was completed. For example, sections of fencing built in the 1970s were grouped in a broad estimate, which we counted as 1970, the earliest possible date. Fencing built in the 1980s gives a specific year, but lists it as an estimate. In those cases, we used the years estimated. Other sections gave a specific year but estimated the month of completion, or indicated a fiscal year but not a calendar date. We limited our analysis to years.

The files CBP provided give an extraordinarily detailed look at fencing that makes up the U.S-Mexico border. However, some elements were not included. Most notably, the digital map provided the length of each section of fencing in an “internal unit” — something other than feet or meters — which we were unable to convert into usable measurements.

Instead, using open source digital mapping software, we were able to extract the length in feet for each section of wall. Although the process is technically difficult — adjusting for the curvature of the Earth on a 2D projection requires some fancy math — the program was able to easily produce an incredibly detailed understanding of the wall.

The shortest section of the wall measured just 2 feet, 5 inches, in Del Rio, Texas. The longest single section, in Yuma, Arizona, spanned almost 15 miles.

Another gap in the data provided is that the map only shows the fencing as it exists today. It doesn’t show what, if anything, existed along the border before the now-standing wall was built.

Additional data

In addition to the detailed map data, we layered decades of other CBP statistics to add context.

Apprehension data on the map shows how many people were detained annually from 1960 through 2016 for attempting to enter the U.S. illegally. Apprehensions do not include deportations handled by U.S. Immigration and Customs Enforcement. They also do not include migrants who tried to enter the U.S. at a port of entry but were declared inadmissible.

The apprehension data CBP provided was broken down by the nine Border Patrol sectors along the Southwest border: two in California (San Diego and El Centro); two in Arizona (Yuma and Tucson); four in Texas (Big Bend, Del Rio, Laredo and Rio Grande Valley); and one that covers all of New Mexico and part of Texas (El Paso).

For our analysis of which Border Patrol sector had the greatest share of apprehensions, we used two primary calculations:

The first is the total number of apprehensions along the U.S.-Mexico border, by year. This information is defined in the document as “Southwest Border Total.”

The second analysis shows each sector’s share of total apprehensions, by year. This calculation was done by dividing apprehensions, by sector and year, with total apprehensions along the border for that year.

Because the building of the wall often was accompanied by an increase in Border Patrol staffing, we also showed on the map the number of agents for each year from 1992 through 2016.

If you have questions or comments about the map, the data or our America’s Wall project, email us at news@kpbs.org.

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