Best-practice manual for geolocation at NHM
Last changed 25.2.2020
Types of grid references in use
Latitude/Longitude (bredde- og lengdegrader)
- 3 different “notations”:
- Decimal degrees, (859,92007N 10,77110E)
- Decimal minutes, (59° 55.204’N 10° 46.266’E)
- Decimal seconds, (59° 55’ 12.3’’N 10° 46’ 16.0’’E)
- Number of decimal digits sets/limits the available precision, e.g. will 5 decimal digits result in a 1 m precision at the latitude of Oslo (60°N), see Table 1
- Decimal degrees-version is preferred by many as it has least posibilities for errors; two simple, decimal numbers, no signs for degrees, minutes or seconds, no mix-up of number of digits for easting or northing, and you may even get rid of the N/S and E/W letter by using negative numbers for southerly and westerly positions
a) Magnitudes of change in Deg, Min & Sec
Distance in meters between points 1 degree/minute/second apart in NS/EW direction, respectively, at different latitudes.
Values calculated for EW differences; NS differences are constant and equal to EW at Latitude = 0. Green coloumns indicate desired minimum precision
- Projected, metric
- Consists of zone, belt, easting and northing (32V 599018 6643834)
- Easting should always contain 6 digits, and within mainland Norway the Northing should always contain 7 digits
- OBS! Uncertainty can thus not be indicated by adjusting the number of digits
MGRS - Military Grid Reference System
- Projected, metric
- Based on UTM: the leading 1-2 digits in Easting and Northing are replaced by a 100,000-meter square identifier (“kvadrant” in Norwegian)
(32V NM 99018 43834)
- An MGRS reference refers to a square area (as opposed to UTM and Lat/long which refer to a point), and the coordinates indicate the lower left (south-western) corner of that square (when e.g. Artskart and GBIF plot MGRS-points to a map, calculations are done so that the point is placed in the middle of the square)
- Uncertainty can be indicated by adjusting the number of digits used for easting and northing; the fewer numbers, the larger the sides of the square will be:
5 digits: 1x1 m square
4 digits: 10x10 m square
3 digits: 100x100 m square
2 digits: 1x1 km square
1 digit: 10x10 km square
- OBS! MGRS coordiantes may have one or more leading 0 in easting and/or northing and this can disappear in excel in some situations
- There are normally equal number of digits in the two groups of numbers in MGRS, but in the lichen collection at NHM it is legal to use e.g. four + three digits, to indicate a rectangle.
Which type to use
Most of NHM’s databases (Musit, Corema, GBIF) accepts all three types of coordinates.
MGRS could be advantageous in the field when one investigates 1 km2 routes.
Other than that, Lat/Long decimal degrees coordinates are in general preferred as these give the least opportunities for errors.
Definition: a defined reference system used as basis for geographical coordinates.
This is an essential part of any map reference!
WGS84 is currently the most widely used standard, and is used e.g. by the GPS-system and in the UTM system.
EUREF89 (also referred to as ETRS89, e.g. in Corema) is used in the “official” Norway, e.g. by Statens Kartverk. This is almost identical to WGS84, with differences in the centimeter range.
At NHM, both MUSIT and Corema will default to WGS84 if no datum is entered.
Accuracy - Uncertainty
The term accuracy often includes both Precision and Trueness,which both have to do with the uncertainty in the estimate of the reference.
Uncertainty in the estimate of the coordinates themselves. Is related to e.g. how good satelitte-cover your GPS has, or how high resolution your map has. Typically given as e.g. +/- 9 m.
Trueness (Norwegian: “Utstrekning”)
Distance between the point referred to by the coordinates and the actual locality. This mat be very small if e.g. every plant collected is given its own set of coordinates, but may be e.g. the extent of the sampling area if only one set of coordinates is given for all plants collected during a day’s work. In the latter example one would typically register the center of the sampling area (with a high precision, e.g. +/- 3 m) and then report the distance to the farthest sampling points (i.e. the radius of a circle including all sampling points) as the Trueness.
Both of these measures are important for our samples and should always accompany any grid reference.
Corema supports registration of both trueness and precision. In MUSIT there are fields for both Precision and Accuracy. DarwinCore have terms for both (Precision = coordinatePrecision, Trueness = coordinateUncertaintyInMeters)
A general rule is to not delete digits from a reference, or use number of digits to convey information about uncertainty, but rather use Precision and Trueness for this purpose.
Finding coordinates after sampling
Look at a map
- digital older maps can be found on www.kartverket.no
- search for locality or browse and click in map
- choose “i” in circle appearing around marker
- choose preferred coordinate-system in menu (MGRS not an option)
- you can also enter coordinates to check where they point to, use the XY-button
- search for locality or browse and click in map
- coordinates appear at the bottom of the window OR
- select “pin” tool and position the crosshair to create a waypoint, containing the coordinates
- change coordinate type in Verktøy\Alternativer...
- lat long desimal degrees
- right-click in map and choose “hva finnes her?”/”What’s here?”
- Obs! Does not always give accurate result
- global version of norgeskart.no
- search for localities (contains an extensive number of locality-names)
- enter coordinates to get them plotted on the map
Converting between different formats
One at a time:
Converting Swedish coordinates:
Batch-converting can be done by importing the data to Corema and export them with the format you wish.
Another way to convert batches of coordinates is MapSource, in combination with other programs:
- GPSBabel makes input files for MapSource. (Does not take MGRS as input-format, see below.) See explanation by Lars Erik.
- Geotrans converts between formats, and does take MGRS-format as input (but not decimal-degrees). See explanation by Lars Erik. Obs! Input file must be positioned in the folder with the Geotrans-folder.
- To convert from MGRS-format: convert to “normal” (non-decimal) lat/long in Geotrans, make input file to MapSource with GPSBabel, open in MapSource and change to decimal degrees.
- In MapSource you can see points in a map. It also links to Google Earth.
- OBS! MapSource adds 0's to MGRS-format if fewer than five digits are entered. This moves the point towards a corner of the square in the map.
Check of coordinates
When format is correct you get a response in the echo-field (ekkofelt).
When administrative place (kommune) is entered, you get an error message if the coordinates do not fall within this region (given that the format of the coordinates are correct).
When there is a response in the echo-filed, you can click the google-map button and see in a map where your point is.
Plot entered data in a map:
Search for accessions, e.g. by event
Open an kml-file in Google earth
Selected app’s for android smart phones (with some iPhone examples)
«GPS Status & Toolbox»
Saving and exporting waypoints. Waypoints can be exported (by the help of FileManager or similar) to CSV, KML, GPX or SQL. These can be opened in GoogleEarth, MapSource/BaseCamp, Excel, Notepad.
Drawback: does not have map
Easy to save waypoints (place x and click on marker-button) that are saved in teh file myWaypoints.wpt in the save-folder (default = Androzic/data/).
Can save to different file, which has to be created first, but one then has to click on the waypoint and do some choices (Blyant/Pencil->Advanced->Choose set)
The waypoint-file can be exported (via FileManager or similar) and opened in e.g. GoogleEarth or Notepad.
WPT is the only export-format.
Drawback: all old WPs in the file will be included if it is not actively emptied before start (Manage, rightclick on Set and choose Clear)
«Norgeskart» («Maps of Norway»)
Very good maps for Norway. Downloaded maps stay on your phone for two weeks.
Cannot save waypoints, but waypoints can be imported from file.
A program that follows Garmin-maps and can import and export data to and from a Garmin GPS device.
Update can be downloaded free from Garmin http://www8.garmin.com/support/download_details.jsp?id=209.
Can be used in converting batches of coordianate-data, see Converting above