GeoPosition calculates map projection coordinates from geographic coordinates (latitude, longitude) and vice versa. It can be used to determine a geographic location on a map. For this purpose you need to know the projection system and the datum of the map, which can usually be found on the map.
The geographic location may be received by the location service of the device or entered manually.
The app provides the following features:
- Conversion from geographic coordinates to map projection coordinates and vice versa
- Display and conversion of the current location received by the location service of the device
- Calculation of the distance of the entered coordinate to the current location
- Display the coordinate on the map view
- Display the distance from the coordinate to the current position on the map view
- Entering the coordinate by moving the pin on the map view
- Sending the coordinate via E-Mail or SMS
- When the coordinate is received via E-Mail, GeoPosition or Google Maps may be started tapping on the embedded link in the E-Mail
- Saving the coordinate with a note
- Exporting the saved coordinates as CSV file
The Coordinate Page
On the coordinate page the geographic coordinate and map projection coordinate may be entered or the location service of the device may be enabled to receive the geographic coordinate. When one coordinate is changed, the other is calculated.
The Geographic Coordinate
In the upper part of the coordinate page, the geographic coordinate (latitude, longitude) may be entered in the following formats:
The different parts of the formats have the following meaning:
- ddd - are the degrees in the range of 0 to 90 for the latitude and 0 to 180 for the longitude
- mm - are the minutes in the range of 0 to 59
- ss - are the seconds in the range of 0 to 59
- The part after the period is the fractional part of the unit
The switch beside the latitude selects the northern N or southern S hemisphere, the switch beside the longitude selects east E or west W of prime meridian. The altitude cannot be entered, because it is irrelevant for the calculation. The altitude is only shown, when the location service is active (see below).
The switch beside the label Distance can be used to switch the distance calculation between the entered coordinate and the current position on and off. When the distance calculation is switched on, the geographic coordinate of the current position and the horizontal accuracy is shown below the distance. On the Map page a line is drawn between the entered coordinate and the current position and the distance is displayed beside the line.
The switch beside the label Use current position can be used to receive the coordinate from the location service of the device. When this service is enabled, the fields to enter the coordinate manually are disabled. The altitude of the current position is displayed and the horizontal and vertical accuracy. Furthermore the indicator left to the switch shows the horizontal accuracy of the received position: red means no signal or an accuracy lower than 100 m, purple means an accuracy between 10 and 100 m, green means an accuracy better than 10 m.
The unit of measurement of the altitude, distance and accuracy is meter (m) or feet (ft), depending on the region format selected in the international settings. Geographic coordinates are considered to be based on the datum (or geodetic system) World Geodetic System 1984 (WGS84).
The Map Projection Coordinate
In the lower part of the coordinate page, the map projection coordinate may be entered. The map projection system (see below) and the datum are selected tapping on the first two fields. The map datum (or geodetic system) determines the polar and equatorial radius of the earth ellipsoid and the distance and rotation of the coordinate system related to the reference datum WGS84. The web sites GeoRepository and NGA Datum Overview of the National Geospacial Agency contains several datum lists with the regions and countries where the datums are used.
The meaning of the following fields to enter the easting, northing or coordinate value and to select the latitude/longitude zone depend on the selected projection system. The field beside the easting field shows the longitude zone and the field beside the northing field shows the latitude zone, also denoted as latitude band. When one of the zone fields is tapped, the zones can be selected. When the selected projection system doesn't support a latitude or longitude zone, the related zone field is disabled. When the projection system defines a special format for its coordinates (e.g. MGRS) these fields are replaced by one field to display and enter the coordinate.
The following map projection systems are supported:
When UTM/UPS is selected the UTM (Universal Transverse Mercator) projection system is used for latitudes between 80°S and 84°N and the UPS (Universal Polar Stereographic) projection system is used for the pole regions.
UTM defines longitude zones with a width of 6° numbered from 1 to 60 starting at 180°W and ending at 180°E. The easting is the distance from the center meridian of the longitude zone measured in meters. To avoid negative easting values, a so called "false easting" of 500,000 meters is added. So, for the datum WGS84 longitude 3°E results in the longitude zone 31 and an easting of 500,000 meters.
UTM defines latitude bands with a height of 8° starting with the letter "C" at 80°S, increasing up the english alphabet until "X", omitting the letters "I" and "O". The last latitude band, "X", has a height of 12°, so it ends at 84°N. In the northern hemisphere, northing is the distance from the equator measured in meters. In the southern hemisphere northing is the distance from the equator subtracted from the so called "false northing" of 10,000,000 meters. So, latitude 0°N results in a northing of 0 meters with the latitude band "N" and latitude 0°S results in a northing of 10,000,000 meters with the latitude band "M".
When a geographic coordinate is calculated from an UTM coordinate, the latitude band is only used to determine the northern/southern hemisphere. So, when you don't know the exact latitude band character of the UTM coordinate just select a letter from "C" to "M" for a coordinate in the southern hemisphere and a letter from "N" to "X" for the northern hemisphere. Then the latitude can be determined by the northing (distance from the equator).
UPS doesn't define a longitude zone, so this zone is always 0. The latitude band character defines the hemisphere for UPS coordinates as follows: "A" for south/west, "B" for south/east, "Y" for north/west and "Z" for north/east. UPS defines the center of the coordinate system at the poles with a northing and easting of 2,000,000 meters.
Valid geographic coordinates for UTM/UPS range from 90°S/180°W to 90°N/180°E. Easting values range from 0 to 3,500,000 meters and northing values from 0 to 10,000,000 meters.
Das Gauß/Krüger Projektionssystem ist, wie UTM eine transversale Mercatorprojektion. Es definiert keine Zone für die südlich-nördliche Richtung aber Zonen für die östlich-westliche Richtung. Diese Zonen haben eine Breite von 3° und werden von 0 bis 119 von West nach Ost durchnummeriert. Der Mittelmeridian der Zone 0 ist der Nullmeridian, der Mittelmeridian der Zone 119 liegt bei 3°W. Der Rechtswert ist der Abstand vom Mittelmeridian der Zone in Metern. Um negative Rechtswerte zu vermeiden, wird zum Abstand ein so genanntes „false easting“ von 500.000 m addiert. Somit erhält man mit dem Datum WGS84 für die Länge 3°O die Zone 1 und einen Rechtswert von 500.000 m. Der Hochwert ist der Abstand vom Äquator in Metern. Auf der südlichen Halbkugel sind die Hochwerte negativ.
Valid geographic coordinates for Gauß/Krüger range from 90°S/180°W to 90°N/180°E. Easting values range from 0 to 1,000,000 meters and northing values from -11,000,000 to 11,000,000 meters.
The Greek Geodetic Reference System 1987 (GGRS87) is a transverse mercator system, which is used in Greece. It is also denoted as "Hellenic Geodetic Reference System 1987" (HGRS87). GGRS87 is usually used together with th datum "GGS87, Greece". It defines no zones, because the projection is defined for coordinates stretching from 6° east to 6° west from the center meridian at 24° east and so all coordinates are mapped to one zone. The value range of geographic coordinates stretches from 33°N/18°O to 42°N/30°O. Valid easting values range from -62,000 m to 1,100,000 m, valid northing values from 3,650,000 m to 4,700,000 m. The coordinate system is used for example on maps from the publisher Anavasi.
The projection system Luxembourg Reference Frame (LUREF) is a transverse mercator system, which is used in Luxembourg. The coordinate system is usually used together with the datum "LURES" (Luxembourg Reference System) or "Luxembourg NT" (which are equivalent). The valid range of geographic coordinates stretches from 49°18'N/5°30'O to 50°18'N/6°36'O. Valid easting values range from 31,500 m to 111,000 m, valid northing values from 40,800 m to 152,000 m.
The Mercator projection system defines no zones. Easting is the distance from the prime meridian in meters. For the western hemisphere easting values are negative. Northing is the distance from the equator in meters. For the southern hemisphere northing values are negative.
Mercator coordinates cannot be calculated for the north or south pole and are strongly distorted near the pole regions. So, valid geographic coordinates for Mercator range from 89°S/180°W to 89°N/180°E. Easting values range from -21,000,000 to 21,000,000 meters and northing values from -31,000,000 to 31,000,000 meters.
The Military Grid Reference System (MGRS) defines a special format for UTM/UPS coordinates. A single coordinate field is used to display and edit the MGRS coordinate in the following format:
zzb cr xxxxx yyyyy
The different parts of the formats have the following meaning:
- zz - is the UTM longitude zone number from 1 to 60. For UPS coordinates this zone number is omitted.
- b - is the UTM/UPS latitude band letter from "A" to "Z" without "I" and "O". The letters "A", "B" and "Y", "Z" are used for UPS coordinates at south and north pole.
- c - is the column letter, which is used to identify the 100,000-meter square in horizontal direction. The valid letters depend on the zone.
- r - is the row letter, which is used to identify the 100,000-meter square in vertical direction. The valid letters depend on the zone.
- xxxxx - is the numerical location in horizontal direction within a 100,000-meter square measured from the left edge of the square in meters.
- yyyyy - is the numerical location in vertical direction within a 100,000-meter square measured from the bottom edge of the square in meters.
The number of digits of the numerical locations in horizontal and vertical direction must be between one and five and equal. The number of digits determine the precision of the coordinate between 1m and 10km. Spaces between the parts are ignored and not necessary. They are just used for a better readability.
Swiss Grid CH1903+(LV95)/CH1903(LV03-M)
Die Swiss reference systems CH1903+(LV95) and CH1903(LV03-M) is a oblique conformal cylinder projection (Mercator projection) with the center at the old observatory in Bern with the coordinates 7°26'22.50" east / 46°57'08.66" north. Coordinates of the system CH1903(LV03-M) are also denoted as Swiss military coordinates or "Schweizer Landeskoordinaten". These projection system are usually used with the datum "Swiss Datum 1903+ (LV95)". Due to the fact, that these projection systems are only used around switzerland valid geographic coordinates range from 45°10'N/4°E to 49°N/11°E. Thus negative easting and northing values are avoided.
The center of the projection system CH1903+(LV95) is defined with an easting of 2,600,000 and a northing of 1,200,000 meters. Easting values range from 2,300,000 to 2,900,000 meters and northing values from 1,001,000 to 1.450.000 meters.
The center of the projection system CH1903+(LV95) is defined with an easting of 600,000 and a northing of 200,000 meters. Easting values range from 300,000 to 900,000 meters and northing values from 1,000 to 450.000 meters. For this system the same transformation formulas are used as for CH1903+(LV95). The only difference are the easting and northing values of the center of the projection. So there may occur distortions of about 1.6 meters with this system.
The Map Page
The map page shows the coordinate on a map view. When you tap on the pin head, an annotation with the geographic and map projection coordinate is displayed. When you tap on the pin head and keep your finger on the pin head you may move the pin to another location and so change the coordinate. This is only possible, when the coordinate is not currently received from the location service of the device (see Use current position switch on the coordinate page). When the location service is active, the position of the pin is adjusted when the coordinate has been changed. When distance calculation is switched on, a line is drawn between the entered coordinate and the current position, and beside the line the distance is shown (see Distance switch on the coordinate page).
The Send Page
The send page can be used to send the coordinate via SMS or E-Mail. When the page is selected the current coordinate is used to create the message. The message is not changed, when the coordinate is changed in the background by the location service. The message contains links, which can be used by the receiver to start GeoPosition or Google Maps with the coordinate.
Saving and Exporting of Coordinates
On the page More coordinates can be saved and saved coordinates can be displayed, selected, deleted and exported. When a coordinate is saved, the data of the current coordinate is shown. At the top a label must be entered for the coordinate. The label is displayed in the coordinate list. Below a note may be entered for the coordinate. Don't forget to tap Save before you leave the page, otherwise your input is not saved.
The Coordinate List
In the coordinate list the saved coordinates are displayed with their label and the last save date. In the search field you can search for the coordinate label. When you tap on a coordinate, the details of the coordinate are shown. On the detail view you may select the coordinate as current coordinate for the other pages of the app or modify the label or the note and save the changes. When you switch in edit mode on the coordinate list view, one ore more coordinates may be selected and deleted. To leave the edit mode tap on Cancel. Single coordinates may also be deleted by wiping.
When you tap Export on the coordinate list view, the saved coordinates are exported to a CSV file (Character Separated Value file) with the name GeoPosition.csv. The file may be transferred to a computer with iTunes. For this purpose, connect the device to the computer, start iTunes and show the apps of your device. Under File Sharing select the app GeoPosition. Then the file is shown for the download. The file may be opened with spreadsheet applications like Numbers or Excel. The semicolon (;) is used as separator in the CSV file. Therefore the semicolon may not be entered in the label or the note for the coordinate. Latitude and longitude of the geographic coordinate are written in decimal format into the CSV file. For the western and southern hemisphere the values are negative. For MGRS coordinates, the column Coordinate contains the projection coordinate in MGRS format.