Here is a breakdown of the equations you need for your conversion.
Once you have a position from GPS in latitude, longitude, and altitude, you will be able to convert this to other units of measurement. This is useful if you want to know the relative distance between two points in metres or, as in your case, feet.
You can use the following conversion to find the distance between two GPS located points. This conversion assumes that the Earth is a sphere but that the distances between points are small enough that the ground can be considered flat.
Latitude and longitude [in decimal degrees] = degrees + (minutes/60) + (seconds/3600)
Latitude difference in meters = latitude difference in decimal degrees x 111,300 m/deg
Longitude difference in meters= longitude difference in decimal degrees x 85,300 m/deg (near 40° N)
Total difference between two points = square root of: (longitude difference2 + latitude difference2 + altitude difference2)
Some GPS software will make this conversion for you. However, it is useful to know how this conversion is made and to have an intuitive feel for what your GPS positions mean.
Frequently it is desirable to use coordinates that are nearly Cartesian in the field. The UTM (Universal Transverse Mercator) projection is frequently used in this case. The projection is a sideways Mercator projection 6 degrees wide along a meridian; positions are expressed as northing and easting and are meters (or kilometres) north of the equator or east of the central meridian (plus 500 km). The declination of northing axis relative to true north is shown on USGS topographic maps, as are the UTM coordinates. Many newer USGS maps show the UTM grid across the map. Nearly all commercial GPS receivers will display UTM coordinates in addition to or instead of latitude and longitude.
Once you have a position from GPS in latitude, longitude, and altitude, you will be able to convert this to other units of measurement. This is useful if you want to know the relative distance between two points in metres or, as in your case, feet.
You can use the following conversion to find the distance between two GPS located points. This conversion assumes that the Earth is a sphere but that the distances between points are small enough that the ground can be considered flat.
Latitude and longitude [in decimal degrees] = degrees + (minutes/60) + (seconds/3600)
Latitude difference in meters = latitude difference in decimal degrees x 111,300 m/deg
Longitude difference in meters= longitude difference in decimal degrees x 85,300 m/deg (near 40° N)
Total difference between two points = square root of: (longitude difference2 + latitude difference2 + altitude difference2)
Some GPS software will make this conversion for you. However, it is useful to know how this conversion is made and to have an intuitive feel for what your GPS positions mean.
Frequently it is desirable to use coordinates that are nearly Cartesian in the field. The UTM (Universal Transverse Mercator) projection is frequently used in this case. The projection is a sideways Mercator projection 6 degrees wide along a meridian; positions are expressed as northing and easting and are meters (or kilometres) north of the equator or east of the central meridian (plus 500 km). The declination of northing axis relative to true north is shown on USGS topographic maps, as are the UTM coordinates. Many newer USGS maps show the UTM grid across the map. Nearly all commercial GPS receivers will display UTM coordinates in addition to or instead of latitude and longitude.
