LAT/LONG TUTORIAL
E911 Wireless Phase 2 introduces new data to the PSAP and the ALI response message. The FCC mandate requires wireless carriers to locate their subscribers dialing 911 and provide the latitude and longitude of the caller’s location to the ALI database. BellSouth will deliver this lat/long information to the PSAP in the ALI response message if the PSAP has implemented the BellSouth “extended” ALI format. Some wireless carriers have chosen to also deliver lat/long of the cell tower or sector centroid if they cannot determine the lat/long of the caller’s location. This is referred to as “phase 1 fallback”. Latitude and longitude are the two grid coordinates by which you can locate any point on Earth. We’ve heard of the terms “lat/long” and “X/Y coordinates” all our lives, but there is a lot of confusion about these. People always say “lat/long” and “X/Y” but few realize that “lat” is NOT the X coordinate and “long” is not the Y coordinate, it is the reverse! If you say “lat/long” you are really saying “Y/X”. This short tutorial was designed to help you understand lat/long and be able to answer questions that our PSAP customers may have since we have become so accustomed to referring to lat/long as X/Y.
WHAT IS LATITUDE?
Latitude, or the Y coordinate, is the distance from the equator. Latitude lines run east and west (horizontal) along the surface of the earth. Every point above the equator is in the “+” (positive) range and anything below the equator is in the “” (negative) range. Coordinate values are preceded by a + or – sign. Think of the standard X/Y chart and think of latitude as being a point along the vertical Y axis. Every area in the United States will have a positive (+) latitude or Y coordinate. All areas within the BellSouth 9 state region will be within the latitude range of approximately +24.5 (Key West, Fl) and +39 (northern KY). Any point on the equator has a latitude of zero while the north pole is +90 and the south pole is 90. Each degree of latitude is divided into 60 equal parts called minutes and each minute can be further divided into 60 seconds, but for the purposes of E911 wireless phase 2, PSAPs don’t need to care about minutes/seconds, only the coordinate values. On the surface of the earth, one degree of latitude is about 69 miles (110 kilometers). Because the earth is not quite a perfect sphere, the distances get slightly greater toward the poles where there is a slight flattening of the earth. Yaxis +90 90 points above and below equator on Yaxis
WHAT IS LONGITUDE?
Longitude, or the X coordinate, is the distance from the Prime Meridian which is in Greenwich, England a borough of London. The earth is divided into two parts, or hemispheres, of east and west longitude. Think of the earth as a globe that is divided into 360 equal slices (180 west and 180 east of Greenwich). The lines between the slices on the globe are called meridians, thus the term “prime meridian” for the meridian that is the starting point, represented by +000.000000 longitude. All points along the prime meridian have a longitude of +000.000000. Longitude lines run north and south along the surface of the earth. Anything east of the Prime Meridian is in the “+” (positive) range and anything west of the Prime Meridian is in the “” (negative) range. Think of the standard X/Y chart and the longitude being a point on the horizontal X axis. All areas within the United States will have a negative () longitude or X coordinate. All areas within the BellSouth 9 state region will be within the longitude range of approximately 75.6 (Cape Hatteras, NC) and 95 (western LA). The space between two meridans is greatest at the equator, about 69 miles (111 kilometers). This space narrows as the meridians approach the north and south poles, so the distance between meridians is not constant. For example, a degree of longitude at New Orleans, LA is about 60 miles (97 kilometers) while at Winnipeg, Canada, a degree of longitude is less than 45 miles (72 kilometers). Xaxis 180 0 +180
QUIZ TIME!
In review, you know the latitude is the coordinate that runs up and down along the vertical Y axis. The longitude is the coordinate that runs right and left along the horizontal X axis. From what we learned in school, the point on the X/Y diagram where the X axis and Y axis meet is 0,0. So here is your quiz question: what point on the earth has a lat/long of +00.000000, +000.000000? The answer is the point where the Prime Meridian meets the equator. So where is that? You have a general idea of where the equator is (the horizontal “belt” line around the center of the earth). And you now know the Prime Meridian runs vertically through a borough of London called Greenwich. If you follow the Prime Meridian down to where it meets the equator, you better have a raft because you are in the Atlantic Ocean just off the coast of Africa!
BellSouth Specifications
BellSouth Technical Reference TR73528, Issue 6 contains the specifications of the ALI response message sent to the PSAP. This TR describes the lat/long data as follows: Optional Longitude/X coordinate: 11 characters with format +000.000000 (or spaces) Optional Latitude/Y coordiante: 10 characters with format +00.000000 (or spaces) The TR describes the format of these fields in this manner to demonstrate that the coordinate values are preceded by a sign (+/) followed by a numerical value which includes 2 or 3 significant digits, a decimal point, then 6 more numerical digits. The longitude/X coordinate will contain 3 significant digits before the decimal point while the latitude/Y coordinate will contain 2 significant digits before the decimal point. As we learned above, the longitude/X coordinate for all locations within the BellSouth region will always be in the negative () range while the latitude/Y coordinate will always be in the positive (+) range. Note that the TR also indicates that the lat/long are OPTIONAL data fields. WireLINE ALI responses will not have lat/long values and some wireLESS ALI responses will not have lat/long values. In these cases the lat/long data in the ALI response sent to the PSAP will contain spaces in the entire data fields (no +/, no decimal, just all spaces). The BellSouth TR also lists the sequence within the ALI response message to the PSAP as X/Y (Long/Lat). So be careful when discussing X/Y or Lat/Long with the PSAP customers and be sure they understand that the data is in the sequence of X/Y but in reality, it is LONG/Lat.
Extra Credit
If you want to earn extra credit and have the information in this tutorial make more sense, you can play around with a simple tool to help you “visualize” lat/long. You may have used MapQuest at one time or another to display a map of an address you were trying to find. Well MapQuest also allows you to display a map when you supply the lat/long coordinates! Open your Internet Explorer (IE) browser and enter the URL: www.mapquest.com Click on the “map” ICON at the top (picture of a globe). Then click “lat/long” on the left of the screen and you will get a screen where you can enter the coordinates in decimal (not degrees). Remember that all points in the BellSouth region fall between latitude +24.5 (Key West) and +39.0 (northern KY) and longitude 75.6 (Cape Hatteras, NC) and 95.0 (western LA). Try varying the combination of lat/long within these ranges to see where it lies within the BellSouth region. Don’t forget the minus sign when you enter the longitude! When you get the map displayed you can zoom out to see a larger view of where the point is. Try lat/long of 0/0 and see what displays. If you get a map that’s all blue, better have your raft handy because your in the water! Zoom all the way out on the map and see how far it is to reach land.
Summary
In summary, you now know that latitude runs up and down the vertical Y axis with all points above the X axis denoted as postivie (+) and all points below the X axis denoted a negative (). On the earth think of the X axis as the equator. Likewise you now know that longitude runs right and left along the horizontal X axis with all points to the right of the Y axis denoted as positive (+) and all points to the left of the Y axis denoted as negative (). On the earth, think of the Y axis as the Prime Meridian running north/south through London/Greenwich. You also learned that all points within the BellSouth region fall in the range: Latitude: +24.5 through +39.0 (positive, north of the equator) Longitude: 75.6 through 95.0 (negative, west of the Prime Meridian) And finally, you learned that you can query on lat/long using MapQuest but if you get a map returned that is all blue, you’re in the water! Bill Marczak, ENP
