today we will talk about the earth and the geographic coordinate system now before looking at more advanced concepts of air navigation we must first look at the place where we are going to navigate which is the earth the earth is the third planet from the sun in the solar system and its shape is almost spherical since it has a polar diameter of 12 714 kilometers and an equatorial diameter of 12 756 kilometers so as we can see the earth is slightly flattened at the poles and elongated at the equator which gives it an oval shape known as an oblate spheroid the reason why the earth has this shape has to do with the centrifugal force it experiences when it rotates on its own axis this movement is called rotation and it takes around 24 hours for the earth to make a complete revolution around its own axis now the points through which this axis of rotation passes are known as the north and south poles which are often used as reference in navigation on the other hand the side towards which the earth rotates is known as east while the opposite side is known as west now apart from this rotational movement the earth also moves around the sun in a movement called translation it takes around 365 days for the earth to make a complete elliptical orbit around the sun and the orbital plane in which the earth moves is known as the ecliptic now it is important to mention that the earth's axis of rotation is inclined in relation to the ecliptic plane this inclination is around 23.5 degrees as we can see and it is responsible for the seasons throughout the year and other phenomena such as the solstices and equinoxes these phenomena occur because the solar radiation received by a certain area of the earth changes throughout the year which has significant implications for weather conditions however we will not go into detail with this now that we know some basic facts about the earth we can now look at the cardinal points the main ones are north south east and west these points are used for orientation and to express directions on the earth's surface so let's define them east represents the direction in which the earth rotates and therefore the west represents the opposite direction to the earth's rotation the north is the pole on the left when we are facing east and the south is the pole diametrically opposite to the north pole now apart from these there are other points known as quadrantal points which are the intermediate directions between the four main cardinal points these quadrantal points are northeast southeast southwest and northwest and at the same time these quadrantal directions can be divided even further for example between north and northeast we find the north northeast between the northeast and the east we find the east northeast and so on so as previously mentioned this system can be used to express the direction to a point in relation to another point let's look at an example here we have two points on the earth's surface so if we want to express the relative direction from the red point to the green point using the cardinal direction system then we can say that the green point is to the northeast of the red point however if we want to express the relative direction from the green point to the red point then the direction will be the opposite since as we can see the red point is to the southwest of the green point the same happens with these other two points in this case for someone located at the orange point the yellow point is to the north while for someone at the yellow point the orange point is to the south so as we could see the cardinal point system is used to express relative directions rather than for expressing exact positions on the earth for that we need a position reference system where each position is precisely and unambiguously defined on the earth's surface an example of a position reference system is the cartesian system in this case two reference axes are used with which it is possible to express any position precisely by means of cartesian coordinates for example if we want to express the position of this red point we can observe that it is located at four units on the x-axis and at three units on the y-axis so the cartesian coordinates for this point would be x4 and y3 so using these axes as reference we can accurately and unambiguously express the position of any point within the scale as we can see here however something important to keep in mind is that this cartesian system only works on a two dimensional surface so it could not be correctly applied to the earth since it is a three-dimensional sphere however despite this the system used on earth is quite similar since it uses two axes as reference and is known as the geographic coordinate system the geographic coordinates are a position reference system that allows any position on the earth to be expressed in terms of latitude and longitude which are equivalent to the units of the x and y axis in the cartesian system however the difference is that since the earth is spherical instead of using linear units as in the cartesian system angular units are used but we will look at this in detail later now the important thing is to know that latitude is used to express how far north or south a point is while longitude is used to express how far east or west a point is let's take a closer look at the reference axes used in this system as in the cartesian system two reference axes are used from which latitude and longitude are measured these axes are the equator line and the greenwich meridian now in order to understand how this reference axes are defined first we have to look at the definition of great circle and small circle a great circle is a plane on a sphere which passing through its center divides it into two equal parts as we can see in this example it is important to note that we can divide the earth in any direction as long as it is divided into two equal parts we will obtain a great circle on the other hand a small circle is a plane on a sphere which do not passes through its center and therefore divides it into two unequal parts so in other words any circle on the earth that is not a great circle will be a small circle here we can see some examples of great and small circles with this in mind the equator line is defined as a great circle perpendicular to the earth's axis of rotation which divides the earth into two equal hemispheres and if we compare it to the cartesian system it would be the equivalent to the x-axis on the other hand the greenwich meridian also known as the prime meridian is defined as a semi-great circle that joins the north and south poles and passes through greenwich in england this would be the equivalent to the y-axis in a cartesian system with this we have the two reference axes that are used in the geographic coordinate system however these axes are also used to divide the earth into four hemispheres this way the equator line divides the earth into the northern and southern hemispheres while the prime meridian divides it into the western and the eastern hemispheres now due to the size of the earth it is common to use secondary reference lines known as parallels and meridians the meridians are semi-great circles that join the north and south poles and they are measured in degrees in relation to the prime meridian these lines are used as a reference for the longitude on the other hand the parallels are small circles parallel to the equator line which are measured in degrees in relation to the equator line these lines are used as a reference for the latitude the resulting grid defined by the parallels and meridians is known as the geographic graticule or grid which is often found on maps and charts having seen all this let's now look at the definition of latitude and longitude the latitude is defined as the angular distance between the equator line and a point on the earth's surface measured in degrees minutes and seconds of arc so in simpler words it measures how far north or how far south a point is now as we can see in the image on the left the latitude is actually an angle measured from the center of the earth where the equator line represents zero degrees in this order of ideas latitude equals zero degrees at the equator and it increases gradually until 90 degrees at each of the poles so then we say that the latitude goes from 0 to 90 degrees north or south on the other hand the longitude is defined as the angular distance between the prime meridian and a point on the earth's surface measured in degrees minutes and seconds of arc so in simpler words it measures how far east or how far west a point is now as we can see in the image on the left longitude is actually an angle measured from the center of the earth where the prime meridian represents zero degrees in this order of ideas longitude equals zero degrees at the prime meridian and it increases gradually until 180 degrees in the opposite side of the planet so then we say that longitude goes from 0 to 180 degrees east or west in these other images we can more clearly see why it goes from 0 to 180 degrees either side in summary then here we can graphically see how latitude and longitude are measured and why angles are used instead of linear units with all this information we should be able to locate any point on a map by means of the geographic coordinate system here then we have a world map with its respective reference axes so to determine the coordinates of a point on this map we do it in the same way as in a cartesian plane for example let's determine the coordinates for this point before proceeding it is important to mention that the latitude always goes first so then the latitude for this point would be 40 degrees north and the longitude would be 100 degrees west now that we know how to do it let's determine the coordinates of these other points for this other position the latitude would be 20 degrees north and the longitude 40 degrees east for this other one the coordinates would be 10 degrees south and 120 degrees east and finally for this one the coordinates would be 60 degrees south and 40 degrees west now so far it seems that this system is quite accurate however if we zoom in we can see that between each degree of latitude and longitude there is a considerably long distance specifically at the equator the distance between each degree of longitude or latitude is 60 nautical miles this way we can accurately express the positions where the parallels and meridians intersect however if we want to determine the coordinates of an intermediate position we would not be able to do it precisely for that reason in order to increase the accuracy the sexagesimal system is used basically this refers to the use of degrees minutes and seconds of arc in the geographic coordinates here each degree of latitude or longitude can be divided into 60 minutes which is also an angular measurement unit so in simpler words one degree equals to 60 minutes now at the same time a minute of arc can be divided into 60 seconds with these smaller units it is now posable to express positions much more accurately let's look at an example as we can see this scale includes the minutes and seconds of arc between the degrees so this way if we want to express the position of this red point we would say that its latitude is 51 degrees 0 minutes and 15 seconds north while the longitude is 11 degrees 1 minute and 30 seconds east in the same way this other point would be at 51 degrees 2 minutes and 0 seconds north and 11 degrees 1 minute and 0 seconds east with this in mind let's determine the coordinates for the following points the point a has a latitude of one degree now to know if it is north or south we have to look at the arrangement of the parallels they increase as we move up that means that the equator line is below us and therefore we are in the northern hemisphere now in terms of longitude it is at 74 degrees and to know if it is west or east we look at the arrangement of the meridians if they increase to the left that means that the prime meridian is to the right of us which means that we are in the western hemisphere so in summary the coordinates of the point a are 1 degree north and 74 degrees west now the latitude of the point b would be one degree south and the longitude is right in the middle between 73 and 74 degrees west so using the sexagesimal system we would say that it is 73 degrees and 30 minutes west finally for the point c the latitude would be 0 degrees and 30 minutes south and the longitude 72 degrees and 30 minutes west let's now look at a more practical example here we have a map with the corresponding grid of parallels and meridians and we must determine the coordinates for the towns of georgetown fairview and greenville so let's start with georgetown in this case we can deduct that we are in the northern hemisphere since the parallels increase as we move up so the latitude would be 28 degrees and 30 minutes north now we can determine that we are in the western hemisphere since the meridians increased to the left so the longitude would be 74 degrees and 22 minutes west now for fairview the coordinates would be 27 degrees 45 minutes north and 73 degrees one minute west and finally for greenville the coordinates are 29 degrees 21 minutes north and 73 degrees 35 minutes west now the current geographic coordinate system is based on a model called the world geodetic system of 1984 which was established as the standard by the icao for air navigation purposes now it is necessary to develop a geodetic model because the earth is not a perfect sphere and therefore the geoid used as a reference in the coordinate system must be adapted as closely as possible to the actual shape of the earth as we can see in this example in the case of the world geodetic system 84 it has an error of less than two centimeters and is used by current gnss systems such as the gps i hope the information presented in this video was useful if so don't forget to share like subscribe and leave a comment down below thanks for watching you