If you have any questions, do feel free to ask in the comments.  I have not touched this blog in years, perhaps this will light a fire under me to actually add more content.

Cheers!

You can see a 7 minute clip of the map in action (with night vision), here:

I viewed that map in the mid 90′s while working on an archaeological survey in the park. The room was dark allowing the bulbs to stand out, but you could not see the lanscape. Also, the narration was a bit on the long monotone and slow side. While informative, it was boring. Of course, this is from the perspective of someone who consumed large amounts of T.V., video games and comic books growing up. Back in the 1930s this animated map was the cat’s pajamas. That’s right, the map has been in service since the 1930s.

I can certainly understand the Park Service position, it does take up a sizable amount of space (30×30 feet, not including the seating around the map), it is antiquated compared to today’s standards, and I imagine a great deal of effort goes into maintaining it. While I am sorry to see it go, it certainly has value in itself from a cartographic historical perspective. I hope it can find a home, but don’t hate the park service for this decision. The marketability of the map simply is not there in an age of short, fast paced internet ready videos interspersed with action sequences to keep the today’s visitors attention. Thank you Hollywood.

For now, the map will remain in storage until a use can be found for it. They plan on replacing the map with a movie or two which would orient the visitors to the battlefield… no word on whether they will include an updated CGI map like the “Map Room” at Guilford Courthouse National Military Park.

Outside of ZIP codes, there are other methods we like to utilize to map our data. One of these is the use of a grid of some arbitrary size over a large area. Many types of data can be mapped to a grid. Temperature, precipitation, elevation each lends themselves to grids due to their continuous nature over a surface. Sometimes other types of data are projected onto grids that are not necessarily continuous, population, geologic formations and income for instance. Grids are also useful for simply indicating locations of geographic features, and have most often been exploited historically for that purpose.

The grid system is probably the easiest method for determining a location on the surface of the earth.There are two types of grids we like to employ, each has strengths and weaknesses. The first is the most common, the Cartesian Coordinate System.

Cartesian Coordinate System

If you have ever played the board game Battleship, then you understand Cartesian Coordinates. With Cartesian system, each cell in the grid is referenced by a pair of X and Y coordinates and measured using linear distance. Cartesian Coordinates are useful only when tied to some geographic unit, (a datum) usually indicated by the (0,0) X/Y coordinates may be arbitrarily located prior to developing the grid, or fixed to a point of cultural or geographic significance.

Ordinarily, the grid is broken into 4 quadrants, with Quadrant 1 holding all positive coordinates. Aside from being easy to read, the usefulness of this gird is increased since we can keep all coordinates positive by setting the datum ahead of time. The Universal Transverse Mercator grid uses this technique to remove the need for negative x and y coordinates, but that is for another entry.

When using a grid of this nature, we would usually indicate the size of each cell in specific squared units. You might use a 1 square mile grid over a city, or as I utilized years ago-1 square meter grid cells over an archaeological site. In each of these examples, each cell is precisely 1 mile or 1 meter squared. This system works well for small areas of no more than a few dozen miles, but any more and we need to take into consideration the curvature of the earth.

To consider the nature of the curvature of the earth, we need to set a projection that would compensate for the distortion in each grid cell size as you moved north and south. For larger areas, even an equidistant projection would not suffice for a national or continental grid of visually equidistant cells. We need a spatial reference system to which all measurements can relate, the Geographic Coordinate System (GCS) must be employed.

Geographic Coordinate System (GCS)

The GCS is a spherical coordinate system that represents position with angles instead of distances. The system uses a network of latitude and longitude lines or graticule to fix surface positions of features or routes. Unlike the Cartesian Coordinate System, the GCS relies on angular measurements from two reference points, as opposed to linear distance from a single reference point. There are two primary reference points of the GCS: North/South Poles and the Equator.

Illustrated above, you notice the angular measurements start from the center of the earth and extend to the surface from two imaginary lines, the Equator and the Prime Meridian. Those two imaginary lines form the datum for the grid (see image below).

Equator

Starting at zero, any point above the equator is positive, below is negative. This line is fairly easy to understand since it marks widest point of the earth along its rotation, it’s in the middle. The angular distance is measured starting at the equator (0˚) and moving to the poles at 90˚ for the north pole and -90˚ for the south pole. Lines of latitude mark the degrees moving away from the equator. These latitude lines are spaced roughly 60 nautical miles apart (69 statute miles) or 111 kilometers.

Prime Meridian

The prime meridian is an arbitrary demarcation of a line that cleanly divides the earth into two equal east/west hemispheres; the line opposite the prime meridian is known as the International Date Line. The position of this prime meridian passes through Greenwich, England for no other reason than England was the prevailing naval power at the time the line was set in 1851. To the east of the prime meridian, all coordinates are positive; to the west the coordinates are negative. Unlike the equator, there are 180˚ from the prime meridian to the International Date Line. Lines of longitude mark the degrees moving away from the prime meridian. The distance between these lines start at roughly 60 nautical miles at the equator, but gradually move closer together until meeting at the poles.

Like a clock, with 360 hours

The coordinates in a GCS are measured in Degrees/MInutes/Seconds, touched on in the illustrations above. When measuring from the equator, the degrees can be anywhere between -90 and 90; from the prime meridian you have -180˚ to 180˚ which are the same line, roughly the international date line. Each degree is broken down into minutes; minutes are further broken into seconds. Just like a clock, there are 60 minutes in the degree and 60 seconds per minute. These coordinates may be denoted in several ways, two of the most popular are Degree/Minutes/Seconds (DMS) and Decimal Degrees (DD).

Often, these GCS coordinates are expressed much like the Catesian Coordinate System. This can be problematic as they do not represent linear distance. However, for purposes of arithmetical computation, they may be treated as cartesian though they represent an angular measurement from the center of the earth expressed on the surface.

DMS is formatted: 42˚ 22′ 06″. This is the oldest format and historically the most common. Many GPS unists will still display this format since it is “human readable” and makes sense to us. However, it is not readily machine readable for GIS software. Software requires the DD format. Using the example above, the equivalent DD format is: 42.36824. In the decimal format, the GIS can readily digest it, interpret the coordinate as a point and plot it on your map. All coordinates must be in DD format to plot in AWhere as well, whether read in from a spreadsheet or manually entered using the point editor add-in. There are many web conversion tools available, or you may make the conversion or manually using the following formula:

Degrees + (minutes / 60) + (seconds / 3600) = Decimal Degree

42 + 22/60 + 6/3600

42 + .366 + .00166

42.36832 (which is a little off from the web tool due to decimal precision)

Which is best?

That depends solely on what the purpose of the grid is. Using a GCS grid is a better option for global data as you can easily and accurately visualize multiple locations around the world for comparison. The GCS makes no claims at showing equal area grid cells. For smaller projects, over part of a city or small region, a Cartesian grid would suffice; they are easy to set up and you can compare areas and linear distance more easily across the surface. Cartesian grids are less flexible than the GCS, however. There are hybrid grids such as the Universal Transverse Mercator and the State Planes, each set their datums in such as way so as to allow for only positive coordinates, but they are designed for specific areas and are best discussed in a future article.

I am often requested to map or facilitate the mapping of some data by ZIP code. The desired result is a thematic choropleth (or heat map) of sales/customer data for these ZIP code divisions . The request itself is not unreasonable, but filled with difficulties inherit to the nature and history of the US ZIP code system.

The problems mapping data by ZIP code areas ultimately originate with the idea that we live in a ZIP code. The very idea of a ZIP code area is ingrained in our minds as these static geographic units with definite boundaries that divide people into manageable groups-this concept even creeps in from pop-culture, ala Beverly Hills, 90210.

If we live in ZIP code areas, reasoning goes, they are a geographic location on which data can be placed, shaped, analyzed and visualized.

Unfortunately, the USPS does not and will not generate a ZIP codes area map for one difficult to grasp reason, ZIP codes do not represent areas. ZIP codes are assigned to post offices or mail collection spots, hence the availability of coordinate location GIS shapefiles and tables. These points service specific routes for mail carriers. Streets, or the sides or streets, or even a single building (different floors of buildings as well, though rarely) may be covered by different ZIP code which acts as a routing code. ZIP codes, therefore may be thought of as linear and point features, but not areas.

One problem influencing the creation of ZIP code maps is that ZIP codes change every year. As new roads are built and companies grow or shrink, ZIP codes may be added or removed. On average, there are about 43,000 ZIP codes in the United States. If you take out the unique P.O. Box, corporate, military and government agency ZIP codes, you end up with a little over 40,000. About 1000 to 3000 ZIP codes may be removed or added, but the number usually fluctuates around 40k.

These changes mean that any ZIP code maps need to be updated every few months to remain current. While the USPS will produce lists of post offices and ZIP codes, they stay away from delineating boundaries. Therefore ZIP code point maps may be seen as more reliable, since they are based on Post Office or mail collection locations. These ZIP code point maps still need to be updated often.

While the USPS does not distribute a ZIP code area map, there are a few third party sources. The techniques used to create these vary, but one method involves buffering the U.S. street centerlines in a GIS, then assigning each resulting polygon the ZIP code on that street segment. Further work must be done to close up holes, divide polygons along streets centers and deal with ZIP codes “islands” where a route might be discontinuous with a portion bisected by another route. One method I have experimented with is the creation of Voronoi polygons. These voronoi polygons make no claims to match up to any “boundaries”, but create a closed collection of polygons using the post office point locations suitable for colorful thematic maps. Ultimately, any one purporting to offer ZIP code areas as GIS shapefiles have at best an extremely rough estimate.

This is not to say you should never map your data by ZIP codes, but be aware of the factors that may eliminate data or alter the accuracy of your map. Much of this depends on the ultimate purpose of the ZIP codes map. A general reference showing which businesses or homes fall into some ZIP code may not be accurate since the ZIP code boundaries are not officially defined. In the end, you should realize that:

1. ZIP codes are not Areas. They do not represent boundaries on the ground in any real sense that you might think of state or county boundaries.
2. ZIP codes are fluid in that they change periodically.
3. ZIP codes might represent a single building, P.O. box, or U.S. Naval vessel.
4. ZIP codes may not fall into the city they are related to, so any census data tied to them may be erroneous.

Sort of, official ZIP code boundaries, but not really

The USPS, despite many calls to do otherwise, has stayed out of the spatial data business. ZIP codes, in the postal mind were designed to do one job and they they do that job very well. So, the Census Bureau eventually took up the task. Enter, the ZIP Code Tabulation Area.

The US Census Bureau created the ZIP Code Tabulation Area or ZCTA (pronounced zikta) due to frustration with requests to map decennial census data at ZIP code level. A solution was to aggregate census blocks and assign them a ZIP code. This process leads to some problems, such as multiple true ZIP codes points (Post Offices) in each ZCTA polygon (not in all, but several) as illustrated in the map below.

(click map to enlarge)

The purple line represent ZCTA boundaries (ZCTA code in purple font) between Winston Salem and Greensboro, North Carolina. Post Office locations are green triangles, labeled with the ZIP codes they service. Note that in many of the ZCTA boundaries there are 2 to 5 Post Offices, meaning any attempt to treat the ZCTA boundary shapefile as a ZIP code boundary will result in missing data.

The Census Bureau, not to be outdone, also provides ZIP code census statistics especially for these ZCTA boundaries. This data allows researchers and businesses to get some rough approximation of the demographics for some ZIP codes… very rough though. Beware too, there are over 43,000 ZIP codes in the US, while there are just less than 30,000 ZCTA numbers. There will always be missing information when trying to map or something else like customer or business data using a ZCTA map layer.

With ZCTA you should understand, at the very least:

1. The ZCTA was designed for specific census datasets and do not reflect the USPS postal code system.
2. The USPS does not support it.
3. Real ZIP codes are point locations referring to post offices or mail collection spots.

Further Reading on the topic of ZIP codes, ZCTA and related data can be found, here:

An article that shares pretty much the same feelings I do on the matter

About ZIP Code Tabulation Areas

The Census position on ZCTA and ZIP codes

ZCTA FAQ

US Census Cartographic Boundary Files