Lago Ercina

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Coordinates used

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  Spain 1973, 1974,
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Several coordinate systems are used on Spanish maps of various vintages, in caving reports, and on Global Positioning System » (GPS) devices. Here are some notes on the most relevant of these, and on software resources.

Coordinates shown on Speleogroup pages are (as of 2019-07-09) shown using WSG84 coordinates (before that date, EUR50 was shown, and the underlying data are also mostly EUR50).

Longitude and Latitude coordinates

EUR50 (European 1950 »)

This longitude and latitude system is shown on the edges of Spanish 1:25,000 maps (the newer ‘blue series’ and the yellow Picos de Europa hiking maps), the 1:50,000 Spanish Military (Cartografía Militar de España) maps, and others (including the 1:80,000 ‘green’ Picos map).  Longitude is relative to the Greenwich Meridian.

To convert EUR50 to WGS84 coordinates, subtract 4.1″ from N and add 4.8″ to W (an adjustment of ~160m SW).
To convert WGS84 to EUR50 coordinates, add 0.001139° to N degrees and add 0.001333° to E degrees (an adjustment of ~160m NE).

EUR50 uses the Hayford (International) ellipsoid model, and is also used in Austria, Finland, Netherlands, Norway, Sweden, and Switzerland. Also sometimes called ED50.

WGS84 (World Geodetic System 1984 »)

This is a global longitude and latitude system developed since the 1950s, and is the default setting for most GPS devices (it is the reference system for GPS).  In Northern Spain using this datum on maps with EUR50 coordinates would place you roughly 160m NE of the correct position.

WGS84 uses its own (WGS84) ellipsoid model.

Madrid Coordinates

Some reports quote positions using a longitude West of the Madrid Meridian.  Add 3°41′16.6″ (3.688°) to a West longitude to convert to EUR50.

Grid coordinates

UTM (Universal Transverse Mercator »)

This is the grid system used on late 20th century Spanish 1:25,000 maps (the newer ‘blue series’ and the yellow Picos de Europa hiking maps), the 1:50,000 Spanish Military (Cartografía Militar de España) maps, and others (including the 1:80,000 ‘green’ Picos map).  The grid is based on Longitude and Latitude 6° lines using the EUR50 system (see above).

Conveniently, the Picos de Europa is centred in UTM zone 30T, so the zone can be omitted when referring to sites in this zone.  However, in general, Spanish caving locations cover more than the one UTM zone, so UTM is inconvenient (and nowadays mostly obsolete) for describing global locations.

Lambert coordinates »

This is the grid system used on Spanish maps through most of the 20th century, with an origin at the longitude of Madrid.  It was widely used in caving reports of the 1960s and 1970s because the maps available at that time used Lambert coordinates.

Lambert grid references are shown with green ticks at the edges of the 1:50,000 Spanish Military (Cartografía Militar de España) maps as late as 1995, but not on more recent maps (including all the 1:25,000 maps).

To convert Lambert to UTM, in the Picos area, use the approximations:

   utmx = 0.09949 * lambx - 152.70
   utmy = 0.1023  * lamby + 3806.68

where lambx and lamby are four-figure (100m) Lambert X and Y grid references.  The result should be correct to about 100m (only tested for the general Picos area, from Ribadesella to 15km south of Cabezón de la Sal).  For example, the OUCC Report 1975/1976 » gives Cabañuca the Lambert grid reference 5282 9671, which, using these formulae, converts to the UTM grid reference 372.8 4796.0.

Software resources

Our lamb2utm.rex is a Rexx » program that will carry out the Lambert to UTM conversion described above.

MemoWiki », the Wiki used for the Speleogroup web site, supports both EUR50 and WGS84 data with direct linking to Google maps; for example, here is a live link to El Cuevón de la PrunedaN43°22'35" W4°43'18".  (GPS locations can also be shown as UTM coordinates: 30T 360.64 4804.26, or as just a pushpin symbol: 📌.)

MemoWiki uses ll2utm.rex to convert GPS lat-lon coordinates to UTM; this has been checked against GeoTrans (see below) and also for the EUR50 coordinate N43°25'56" W4°50'05" (UTM: 30T 351.62 4810.66) on the Llanes 1:25,000 map (sheet 32–I).

For a general coordinate conversion utility (which is free), try GeoTrans »; both Unix and Windows versions are available.  For more limited code (Java, JS, or PHP) try JScoord ».  For a helpful page on the background with useful links, see Karen Nakamura’s » page.

Expeditions to the Picos de Europa and elsewhere since 1973.
Please e-mail Mike Cowlishaw ( or Bill Collis (
if you have any corrections, suggestions, etc.   See also the SpeleoTrove speleology section ».
The pages and data here are for non-commercial use only.   All content © Speleogroup 1973, 2023, except where marked otherwise.  All rights reserved.  Here is our privacy policy.

This page was last edited on 2019-07-19 by mfc.