Exiv2

It's in Exif.GPSInfo.Altitude

644 rmills@rmillsmm:~/gnu/exiv2/trunk $ exiv2 -pa --grep bar/i --grep alt/i http://dev.exiv2.org/attachments/download/1060/25850685556_622312410d_o.jpg
Exif.GPSInfo.GPSAltitudeRef                  Byte        1  Above sea level
Exif.GPSInfo.GPSAltitude                     Rational    1  1601 m
Xmp.exif.GPSAltitude                         XmpText     6  1601/1
Xmp.exif.GPSAltitudeRef                      XmpText     1  Above sea level
645 rmills@rmillsmm:~/gnu/exiv2/trunk $ 

Thanks Robin for your answer. Exif.GPSInfo.GPSAltitude is not the barometric altitude information, but the GPS information. exiftools shows to Altitude values:

rose@lynx:/home/rose/Txt/projects/Robina(163)$ exiftool Testphotos_Exif/Nikon_1/25850685556_622312410d_o.jpg | grep Altitude
Altitude : 1583 m
GPS Altitude Ref : Above Sea Level
GPS Altitude : 1601 m Above Sea Level

i.e., the difference is about 20 m. This difference could be relevant for us. In /usr/lib64/perl5/vendor_perl/5.24.0/Image/ExifTool/Nikon.pm
I find:

1. 13) Roger Larsson private communication (tests with D200)
2. 14) http://homepage3.nifty.com/kamisaka/makernote/makernote_nikon.htm (2007/09/15)
3. 15) http://tomtia.plala.jp/DigitalCamera/MakerNote/index.asp
4. 16) Jeffrey Friedl private communication (D200 with firmware update)
5. 17) http://www.wohlberg.net/public/software/photo/nstiffexif/
   --
   CHECK_PROC => \&Image::ExifTool::Exif::CheckExif,
   WRITABLE => 1,
   GROUPS => { 0 => 'MakerNotes', 2 => 'Camera' },
   PRINT_CONV => \&FormatString,
   0x0001 => { #2 # the format differs for different models. for D70, this is a string '0210', # but for the E775 it is binary: "\x00\x01\x00\x00"
   Name => 'MakerNoteVersion',
   Writable => 'undef',
   Count => 4,
   --
   },
   0x00c3 => {
   Name => 'BarometerInfo',
   SubDirectory => {
   TagTable => 'Image::ExifTool::Nikon::BarometerInfo',
   ByteOrder => 'LittleEndian',
   },
   --
   Flags => [ 'Binary', 'Protected', 'Drop' ],
   Notes => q{
   this data is dropped when copying Nikon MakerNotes since it may be too large
   to fit in the EXIF segment of a JPEG image, but it may be copied as a block
   into existing Nikon MakerNotes later if desired
   },
   SubDirectory => {
   --
   WRITE_GROUP => 'NikonScan',
   WRITABLE => 1,
   GROUPS => { 0 => 'MakerNotes', 1 => 'NikonScan', 2 => 'Image' },
   ...
   %Image::ExifTool::Nikon::FileInfo = (
   %binaryDataAttrs,
   GROUPS => { 0 => 'MakerNotes', 2 => 'Image' },
   FORMAT => 'int16u',
   0 => {
   --

1. ref PH
   %Image::ExifTool::Nikon::BarometerInfo = (
   %binaryDataAttrs,
   GROUPS => { 0 => 'MakerNotes', 2 => 'Location' },
   0 => {
   Name => 'BarometerInfoVersion',
   Format => 'undef⁴',
   Writable => 0,
   },
   6 => {
   Name => 'Altitude',
   Format => 'int32s',
   PrintConv => '"$val m"', # (always stored as metres)
   ...

If I interpret this correctly, the BarometerInfo starts with an offset of 0x00c3 in MakerNotes. But I have no idea, how can access this information with libexiv2.

Please raise an issue and this will be investigated for a future release.

I filed the issue at http://dev.exiv2.org/issues/1225 .