diff --git a/src/tag.rs b/src/tag.rs
index dc253dd..b24bdf5 100644
--- a/src/tag.rs
+++ b/src/tag.rs
@@ -26,6 +26,8 @@
 
 use std::fmt;
 
+use value::Value;
+
 /// A tag of a TIFF field.
 //
 // This is not an enum to keep safety and API stability, while
@@ -64,6 +66,13 @@ impl Tag {
     pub fn description(&self) -> Option<&str> {
         get_tag_info(self).map(|ti| ti.desc)
     }
+
+    /// Returns the default value of the tag.  `None` is returned if
+    /// it is not defined in the standard or it depends on the context.
+    #[inline]
+    pub fn default_value(&self) -> Option<Value> {
+        get_tag_info(self).and_then(|ti| (&ti.default).into())
+    }
 }
 
 impl fmt::Display for Tag {
@@ -93,7 +102,7 @@ macro_rules! generate_well_known_tag_constants {
         $( |$ctx:path| $(
             // Copy the doc attribute to the actual definition.
             $( #[$attr:meta] )*
-            ($name:ident, $num:expr, $desc:expr)
+            ($name:ident, $num:expr, $defval:expr, $desc:expr)
         ),+, )+
     ) => (
         /// A module that contains Exif tag constants.
@@ -122,15 +131,18 @@ macro_rules! generate_well_known_tag_constants {
         // Use a separate module to avoid name conflicts between
         // const Tag and static TagInfo.
         mod tag_info {
+            use value::DefaultValue;
+
             pub struct TagInfo {
                 pub name: &'static str,
                 pub desc: &'static str,
+                pub default: DefaultValue,
             }
 
             $($(
                 #[allow(non_upper_case_globals)]
                 pub static $name: TagInfo = TagInfo {
-                    name: stringify!($name), desc: $desc };
+                    name: stringify!($name), desc: $desc, default: $defval };
             )+)+
         }
 
@@ -153,174 +165,321 @@ generate_well_known_tag_constants!(
 
     /// A pointer to the Exif IFD.  This is used for the internal structure
     /// of Exif data and will not be returned to the user.
-    (ExifIFDPointer, 0x8769, "Exif IFD pointer"),
+    (ExifIFDPointer, 0x8769, DefaultValue::None,
+     "Exif IFD pointer"),
     /// A pointer to the GPS IFD.  This is used for the internal structure
     /// of Exif data and will not be returned to the user.
-    (GPSInfoIFDPointer, 0x8825, "GPS Info IFD pointer"),
+    (GPSInfoIFDPointer, 0x8825, DefaultValue::None,
+     "GPS Info IFD pointer"),
 
     |Context::Exif|
 
     /// A pointer to the interoperability IFD.  This is used for the internal
     /// structure of Exif data and will not be returned to the user.
-    (InteropIFDPointer, 0xa005, "Interoperability IFD pointer"),
+    (InteropIFDPointer, 0xa005, DefaultValue::None,
+     "Interoperability IFD pointer"),
 
     // TIFF primary and thumbnail attributes [EXIF23 4.6.4 Table 4,
     // 4.6.8 Table 17, and 4.6.8 Table 21].
     |Context::Tiff|
 
-    (ImageWidth, 0x100, "Image width"),
-    (ImageLength, 0x101, "Image height"),
-    (BitsPerSample, 0x102, "Number of bits per component"),
-    (Compression, 0x103, "Compression scheme"),
-    (PhotometricInterpretation, 0x106, "Pixel composition"),
-    (ImageDescription, 0x10e, "Image title"),
-    (Make, 0x10f, "Manufacturer of image input equipment"),
-    (Model, 0x110, "Model of image input equipment"),
-    (StripOffsets, 0x111, "Image data location"),
-    (Orientation, 0x112, "Orientation of image"),
-    (SamplesPerPixel, 0x115, "Number of components"),
-    (RowsPerStrip, 0x116, "Number of rows per strip"),
-    (StripByteCounts, 0x117, "Bytes per compressed strip"),
-    (XResolution, 0x11a, "Image resolution in width direction"),
-    (YResolution, 0x11b, "Image resolution in height direction"),
-    (PlanarConfiguration, 0x11c, "Image data arrangement"),
-    (ResolutionUnit, 0x128, "Unit of X and Y resolution"),
-    (TransferFunction, 0x12d, "Transfer function"),
-    (Software, 0x131, "Software used"),
-    (DateTime, 0x132, "File change date and time"),
-    (Artist, 0x13b, "Person who created the image"),
-    (WhitePoint, 0x13e, "White point chromaticity"),
-    (PrimaryChromaticities, 0x13f, "Chromaticities of primaries"),
-    (JPEGInterchangeFormat, 0x201, "Offset to JPEG SOI"),
-    (JPEGInterchangeFormatLength, 0x202, "Bytes of JPEG data"),
-    (YCbCrCoefficients, 0x211, "Color space transformation matrix coefficients"),
-    (YCbCrSubSampling, 0x212, "Subsampling ratio of Y to C"),
-    (YCbCrPositioning, 0x213, "Y and C positioning"),
-    (ReferenceBlackWhite, 0x214, "Pair of black and white reference values"),
-    (Copyright, 0x8298, "Copyright holder"),
+    (ImageWidth, 0x100, DefaultValue::None,
+     "Image width"),
+    (ImageLength, 0x101, DefaultValue::None,
+     "Image height"),
+    (BitsPerSample, 0x102, DefaultValue::Short(&[8, 8, 8]),
+     "Number of bits per component"),
+    (Compression, 0x103, DefaultValue::None,
+     "Compression scheme"),
+    (PhotometricInterpretation, 0x106, DefaultValue::None,
+     "Pixel composition"),
+    (ImageDescription, 0x10e, DefaultValue::None,
+     "Image title"),
+    (Make, 0x10f, DefaultValue::None,
+     "Manufacturer of image input equipment"),
+    (Model, 0x110, DefaultValue::None,
+     "Model of image input equipment"),
+    (StripOffsets, 0x111, DefaultValue::None,
+     "Image data location"),
+    (Orientation, 0x112, DefaultValue::Short(&[1]),
+     "Orientation of image"),
+    (SamplesPerPixel, 0x115, DefaultValue::Short(&[3]),
+     "Number of components"),
+    (RowsPerStrip, 0x116, DefaultValue::None,
+     "Number of rows per strip"),
+    (StripByteCounts, 0x117, DefaultValue::None,
+     "Bytes per compressed strip"),
+    (XResolution, 0x11a, DefaultValue::Rational(&[(72, 1)]),
+     "Image resolution in width direction"),
+    (YResolution, 0x11b, DefaultValue::Rational(&[(72, 1)]),
+     "Image resolution in height direction"),
+    (PlanarConfiguration, 0x11c, DefaultValue::Short(&[1]),
+     "Image data arrangement"),
+    (ResolutionUnit, 0x128, DefaultValue::Short(&[2]),
+     "Unit of X and Y resolution"),
+    (TransferFunction, 0x12d, DefaultValue::None,
+     "Transfer function"),
+    (Software, 0x131, DefaultValue::None,
+     "Software used"),
+    (DateTime, 0x132, DefaultValue::None,
+     "File change date and time"),
+    (Artist, 0x13b, DefaultValue::None,
+     "Person who created the image"),
+    (WhitePoint, 0x13e, DefaultValue::None,
+     "White point chromaticity"),
+    (PrimaryChromaticities, 0x13f, DefaultValue::None,
+     "Chromaticities of primaries"),
+    (JPEGInterchangeFormat, 0x201, DefaultValue::None,
+     "Offset to JPEG SOI"),
+    (JPEGInterchangeFormatLength, 0x202, DefaultValue::None,
+     "Bytes of JPEG data"),
+    (YCbCrCoefficients, 0x211, DefaultValue::Unspecified,
+     "Color space transformation matrix coefficients"),
+    (YCbCrSubSampling, 0x212, DefaultValue::None,
+     "Subsampling ratio of Y to C"),
+    (YCbCrPositioning, 0x213, DefaultValue::Short(&[1]),
+     "Y and C positioning"),
+    (ReferenceBlackWhite, 0x214, DefaultValue::ContextDependent,
+     "Pair of black and white reference values"),
+    (Copyright, 0x8298, DefaultValue::None,
+     "Copyright holder"),
 
     // Exif IFD attributes [EXIF23 4.6.5 Table 7 and 4.6.8 Table 18].
     |Context::Exif|
 
-    (ExposureTime, 0x829a, "Exposure time"),
-    (FNumber, 0x829d, "F number"),
-    (ExposureProgram, 0x8822, "Exposure program"),
-    (SpectralSensitivity, 0x8824, "Spectral sensitivity"),
-    (PhotographicSensitivity, 0x8827, "Photographic sensitivity"),
-    (OECF, 0x8828, "Optoelectric conversion factor"),
-    (SensitivityType, 0x8830, "Sensitivity type"),
-    (StandardOutputSensitivity, 0x8831, "Standard output sensitivity"),
-    (RecommendedExposureIndex, 0x8832, "Recommended exposure index"),
-    (ISOSpeed, 0x8833, "ISO speed"),
-    (ISOSpeedLatitudeyyy, 0x8834, "ISO speed latitude yyy"),
-    (ISOSpeedLatitudezzz, 0x8835, "ISO speed latitude zzz"),
-    (ExifVersion, 0x9000, "Exif version"),
-    (DateTimeOriginal, 0x9003, "Date and time of original data generation"),
-    (DateTimeDigitized, 0x9004, "Date and time of digital data generation"),
-    (OffsetTime, 0x9010, "Offset data of DateTime"),
-    (OffsetTimeOriginal, 0x9011, "Offset data of DateTimeOriginal"),
-    (OffsetTimeDigitized, 0x9012, "Offset data of DateTimeDigitized"),
-    (ComponentsConfiguration, 0x9101, "Meaning of each component"),
-    (CompressedBitsPerPixel, 0x9102, "Image compression mode"),
-    (ShutterSpeedValue, 0x9201, "Shutter speed"),
-    (ApertureValue, 0x9202, "Aperture"),
-    (BrightnessValue, 0x9203, "Brightness"),
-    (ExposureBiasValue, 0x9204, "Exposure bias"),
-    (MaxApertureValue, 0x9205, "Maximum lens aperture"),
-    (SubjectDistance, 0x9206, "Subject distance"),
-    (MeteringMode, 0x9207, "Metering mode"),
-    (LightSource, 0x9208, "Light source"),
-    (Flash, 0x9209, "Flash"),
-    (FocalLength, 0x920a, "Lens focal length"),
-    (SubjectArea, 0x9214, "Subject area"),
-    (MakerNote, 0x927c, "Manufacturer notes"),
-    (UserComment, 0x9286, "User comments"),
-    (SubSecTime, 0x9290, "DateTime subseconds"),
-    (SubSecTimeOriginal, 0x9291, "DateTimeOriginal subseconds"),
-    (SubSecTimeDigitized, 0x9292, "DateTimeDigitized subseconds"),
-    (Temperature, 0x9400, "Temperature"),
-    (Humidity, 0x9401, "Humidity"),
-    (Pressure, 0x9402, "Pressure"),
-    (WaterDepth, 0x9403, "Water depth"),
-    (Acceleration, 0x9404, "Acceleration"),
-    (CameraElevationAngle, 0x9405, "Camera elevation angle"),
-    (FlashpixVersion, 0xa000, "Supported Flashpix version"),
-    (ColorSpace, 0xa001, "Color space information"),
-    (PixelXDimension, 0xa002, "Valid image width"),
-    (PixelYDimension, 0xa003, "Valid image height"),
-    (RelatedSoundFile, 0xa004, "Related audio file"),
-    (FlashEnergy, 0xa20b, "Flash energy"),
-    (SpatialFrequencyResponse, 0xa20c, "Spatial frequency response"),
-    (FocalPlaneXResolution, 0xa20e, "Focal plane X resolution"),
-    (FocalPlaneYResolution, 0xa20f, "Focal plane Y resolution"),
-    (FocalPlaneResolutionUnit, 0xa210, "Focal plane resolution unit"),
-    (SubjectLocation, 0xa214, "Subject location"),
-    (ExposureIndex, 0xa215, "Exposure index"),
-    (SensingMethod, 0xa217, "Sensing method"),
-    (FileSource, 0xa300, "File source"),
-    (SceneType, 0xa301, "Scene type"),
-    (CFAPattern, 0xa302, "CFA pattern"),
-    (CustomRendered, 0xa401, "Custom image processing"),
-    (ExposureMode, 0xa402, "Exposure mode"),
-    (WhiteBalance, 0xa403, "White balance"),
-    (DigitalZoomRatio, 0xa404, "Digital zoom ratio"),
-    (FocalLengthIn35mmFilm, 0xa405, "Focal length in 35 mm film"),
-    (SceneCaptureType, 0xa406, "Scene capture type"),
-    (GainControl, 0xa407, "Gain control"),
-    (Contrast, 0xa408, "Contrast"),
-    (Saturation, 0xa409, "Saturation"),
-    (Sharpness, 0xa40a, "Sharpness"),
-    (DeviceSettingDescription, 0xa40b, "Device settings description"),
-    (SubjectDistanceRange, 0xa40c, "Subject distance range"),
-    (ImageUniqueID, 0xa420, "Unique image ID"),
-    (CameraOwnerName, 0xa430, "Camera owner name"),
-    (BodySerialNumber, 0xa431, "Body serial number"),
-    (LensSpecification, 0xa432, "Lens specification"),
-    (LensMake, 0xa433, "Lens make"),
-    (LensModel, 0xa434, "Lens model"),
-    (LensSerialNumber, 0xa435, "Lens serial number"),
-    (Gamma, 0xa500, "Gamma"),
+    (ExposureTime, 0x829a, DefaultValue::None,
+     "Exposure time"),
+    (FNumber, 0x829d, DefaultValue::None,
+     "F number"),
+    (ExposureProgram, 0x8822, DefaultValue::None,
+     "Exposure program"),
+    (SpectralSensitivity, 0x8824, DefaultValue::None,
+     "Spectral sensitivity"),
+    (PhotographicSensitivity, 0x8827, DefaultValue::None,
+     "Photographic sensitivity"),
+    (OECF, 0x8828, DefaultValue::None,
+     "Optoelectric conversion factor"),
+    (SensitivityType, 0x8830, DefaultValue::None,
+     "Sensitivity type"),
+    (StandardOutputSensitivity, 0x8831, DefaultValue::None,
+     "Standard output sensitivity"),
+    (RecommendedExposureIndex, 0x8832, DefaultValue::None,
+     "Recommended exposure index"),
+    (ISOSpeed, 0x8833, DefaultValue::None,
+     "ISO speed"),
+    (ISOSpeedLatitudeyyy, 0x8834, DefaultValue::None,
+     "ISO speed latitude yyy"),
+    (ISOSpeedLatitudezzz, 0x8835, DefaultValue::None,
+     "ISO speed latitude zzz"),
+    // The absence of this field means non-conformance to Exif, so the default
+    // value specified in the standard (e.g., "0231") should not apply.
+    (ExifVersion, 0x9000, DefaultValue::None,
+     "Exif version"),
+    (DateTimeOriginal, 0x9003, DefaultValue::None,
+     "Date and time of original data generation"),
+    (DateTimeDigitized, 0x9004, DefaultValue::None,
+     "Date and time of digital data generation"),
+    (OffsetTime, 0x9010, DefaultValue::None,
+     "Offset data of DateTime"),
+    (OffsetTimeOriginal, 0x9011, DefaultValue::None,
+     "Offset data of DateTimeOriginal"),
+    (OffsetTimeDigitized, 0x9012, DefaultValue::None,
+     "Offset data of DateTimeDigitized"),
+    (ComponentsConfiguration, 0x9101, DefaultValue::ContextDependent,
+     "Meaning of each component"),
+    (CompressedBitsPerPixel, 0x9102, DefaultValue::None,
+     "Image compression mode"),
+    (ShutterSpeedValue, 0x9201, DefaultValue::None,
+     "Shutter speed"),
+    (ApertureValue, 0x9202, DefaultValue::None,
+     "Aperture"),
+    (BrightnessValue, 0x9203, DefaultValue::None,
+     "Brightness"),
+    (ExposureBiasValue, 0x9204, DefaultValue::None,
+     "Exposure bias"),
+    (MaxApertureValue, 0x9205, DefaultValue::None,
+     "Maximum lens aperture"),
+    (SubjectDistance, 0x9206, DefaultValue::None,
+     "Subject distance"),
+    (MeteringMode, 0x9207, DefaultValue::Short(&[0]),
+     "Metering mode"),
+    (LightSource, 0x9208, DefaultValue::Short(&[0]),
+     "Light source"),
+    (Flash, 0x9209, DefaultValue::Unspecified,
+     "Flash"),
+    (FocalLength, 0x920a, DefaultValue::None,
+     "Lens focal length"),
+    (SubjectArea, 0x9214, DefaultValue::None,
+     "Subject area"),
+    (MakerNote, 0x927c, DefaultValue::None,
+     "Manufacturer notes"),
+    (UserComment, 0x9286, DefaultValue::None,
+     "User comments"),
+    (SubSecTime, 0x9290, DefaultValue::None,
+     "DateTime subseconds"),
+    (SubSecTimeOriginal, 0x9291, DefaultValue::None,
+     "DateTimeOriginal subseconds"),
+    (SubSecTimeDigitized, 0x9292, DefaultValue::None,
+     "DateTimeDigitized subseconds"),
+    (Temperature, 0x9400, DefaultValue::None,
+     "Temperature"),
+    (Humidity, 0x9401, DefaultValue::None,
+     "Humidity"),
+    (Pressure, 0x9402, DefaultValue::None,
+     "Pressure"),
+    (WaterDepth, 0x9403, DefaultValue::None,
+     "Water depth"),
+    (Acceleration, 0x9404, DefaultValue::None,
+     "Acceleration"),
+    (CameraElevationAngle, 0x9405, DefaultValue::None,
+     "Camera elevation angle"),
+    (FlashpixVersion, 0xa000, DefaultValue::Undefined(b"0100"),
+     "Supported Flashpix version"),
+    (ColorSpace, 0xa001, DefaultValue::Unspecified,
+     "Color space information"),
+    (PixelXDimension, 0xa002, DefaultValue::None,
+     "Valid image width"),
+    (PixelYDimension, 0xa003, DefaultValue::Unspecified,
+     "Valid image height"),
+    (RelatedSoundFile, 0xa004, DefaultValue::None,
+     "Related audio file"),
+    (FlashEnergy, 0xa20b, DefaultValue::None,
+     "Flash energy"),
+    (SpatialFrequencyResponse, 0xa20c, DefaultValue::None,
+     "Spatial frequency response"),
+    (FocalPlaneXResolution, 0xa20e, DefaultValue::None,
+     "Focal plane X resolution"),
+    (FocalPlaneYResolution, 0xa20f, DefaultValue::None,
+     "Focal plane Y resolution"),
+    (FocalPlaneResolutionUnit, 0xa210, DefaultValue::Short(&[2]),
+     "Focal plane resolution unit"),
+    (SubjectLocation, 0xa214, DefaultValue::None,
+     "Subject location"),
+    (ExposureIndex, 0xa215, DefaultValue::None,
+     "Exposure index"),
+    (SensingMethod, 0xa217, DefaultValue::None,
+     "Sensing method"),
+    (FileSource, 0xa300, DefaultValue::Undefined(&[3]),
+     "File source"),
+    (SceneType, 0xa301, DefaultValue::Undefined(&[1]),
+     "Scene type"),
+    (CFAPattern, 0xa302, DefaultValue::None,
+     "CFA pattern"),
+    (CustomRendered, 0xa401, DefaultValue::Short(&[0]),
+     "Custom image processing"),
+    (ExposureMode, 0xa402, DefaultValue::None,
+     "Exposure mode"),
+    (WhiteBalance, 0xa403, DefaultValue::None,
+     "White balance"),
+    (DigitalZoomRatio, 0xa404, DefaultValue::None,
+     "Digital zoom ratio"),
+    (FocalLengthIn35mmFilm, 0xa405, DefaultValue::None,
+     "Focal length in 35 mm film"),
+    (SceneCaptureType, 0xa406, DefaultValue::Short(&[0]),
+     "Scene capture type"),
+    (GainControl, 0xa407, DefaultValue::None,
+     "Gain control"),
+    (Contrast, 0xa408, DefaultValue::Short(&[0]),
+     "Contrast"),
+    (Saturation, 0xa409, DefaultValue::Short(&[0]),
+     "Saturation"),
+    (Sharpness, 0xa40a, DefaultValue::Short(&[0]),
+     "Sharpness"),
+    (DeviceSettingDescription, 0xa40b, DefaultValue::None,
+     "Device settings description"),
+    (SubjectDistanceRange, 0xa40c, DefaultValue::None,
+     "Subject distance range"),
+    (ImageUniqueID, 0xa420, DefaultValue::None,
+     "Unique image ID"),
+    (CameraOwnerName, 0xa430, DefaultValue::None,
+     "Camera owner name"),
+    (BodySerialNumber, 0xa431, DefaultValue::None,
+     "Body serial number"),
+    (LensSpecification, 0xa432, DefaultValue::None,
+     "Lens specification"),
+    (LensMake, 0xa433, DefaultValue::None,
+     "Lens make"),
+    (LensModel, 0xa434, DefaultValue::None,
+     "Lens model"),
+    (LensSerialNumber, 0xa435, DefaultValue::None,
+     "Lens serial number"),
+    (Gamma, 0xa500, DefaultValue::None,
+     "Gamma"),
 
     // GPS attributes [EXIF23 4.6.6 Table 15 and 4.6.8 Table 19].
     |Context::Gps|
 
-    (GPSVersionID, 0x0, "GPS tag version"),
-    (GPSLatitudeRef, 0x1, "North or south latitude"),
-    (GPSLatitude, 0x2, "Latitude"),
-    (GPSLongitudeRef, 0x3, "East or West Longitude"),
-    (GPSLongitude, 0x4, "Longitude"),
-    (GPSAltitudeRef, 0x5, "Altitude reference"),
-    (GPSAltitude, 0x6, "Altitude"),
-    (GPSTimeStamp, 0x7, "GPS time (atomic clock)"),
-    (GPSSatellites, 0x8, "GPS satellites used for measurement"),
-    (GPSStatus, 0x9, "GPS receiver status"),
-    (GPSMeasureMode, 0xa, "GPS measurement mode"),
-    (GPSDOP, 0xb, "Measurement precision"),
-    (GPSSpeedRef, 0xc, "Speed unit"),
-    (GPSSpeed, 0xd, "Speed of GPS receiver"),
-    (GPSTrackRef, 0xe, "Reference for direction of movement"),
-    (GPSTrack, 0xf, "Direction of movement"),
-    (GPSImgDirectionRef, 0x10, "Reference for direction of image"),
-    (GPSImgDirection, 0x11, "Direction of image"),
-    (GPSMapDatum, 0x12, "Geodetic survey data used"),
-    (GPSDestLatitudeRef, 0x13, "Reference for latitude of destination"),
-    (GPSDestLatitude, 0x14, "Latitude of destination"),
-    (GPSDestLongitudeRef, 0x15, "Reference for longitude of destination"),
-    (GPSDestLongitude, 0x16, "Longitude of destination"),
-    (GPSDestBearingRef, 0x17, "Reference for bearing of destination"),
-    (GPSDestBearing, 0x18, "Bearing of destination"),
-    (GPSDestDistanceRef, 0x19, "Reference for distance to destination"),
-    (GPSDestDistance, 0x1a, "Distance to destination"),
-    (GPSProcessingMethod, 0x1b, "Name of GPS processing method"),
-    (GPSAreaInformation, 0x1c, "Name of GPS area"),
-    (GPSDateStamp, 0x1d, "GPS date"),
-    (GPSDifferential, 0x1e, "GPS differential correction"),
-    (GPSHPositioningError, 0x1f, "Horizontal positioning error"),
+    // Depends on the Exif version.
+    (GPSVersionID, 0x0, DefaultValue::ContextDependent,
+     "GPS tag version"),
+    (GPSLatitudeRef, 0x1, DefaultValue::None,
+     "North or south latitude"),
+    (GPSLatitude, 0x2, DefaultValue::None,
+     "Latitude"),
+    (GPSLongitudeRef, 0x3, DefaultValue::None,
+     "East or West Longitude"),
+    (GPSLongitude, 0x4, DefaultValue::None,
+     "Longitude"),
+    (GPSAltitudeRef, 0x5, DefaultValue::Byte(&[0]),
+     "Altitude reference"),
+    (GPSAltitude, 0x6, DefaultValue::None,
+     "Altitude"),
+    (GPSTimeStamp, 0x7, DefaultValue::None,
+     "GPS time (atomic clock)"),
+    (GPSSatellites, 0x8, DefaultValue::None,
+     "GPS satellites used for measurement"),
+    (GPSStatus, 0x9, DefaultValue::None,
+     "GPS receiver status"),
+    (GPSMeasureMode, 0xa, DefaultValue::None,
+     "GPS measurement mode"),
+    (GPSDOP, 0xb, DefaultValue::None,
+     "Measurement precision"),
+    (GPSSpeedRef, 0xc, DefaultValue::Ascii(&[b"K"]),
+     "Speed unit"),
+    (GPSSpeed, 0xd, DefaultValue::None,
+     "Speed of GPS receiver"),
+    (GPSTrackRef, 0xe, DefaultValue::Ascii(&[b"T"]),
+     "Reference for direction of movement"),
+    (GPSTrack, 0xf, DefaultValue::None,
+     "Direction of movement"),
+    (GPSImgDirectionRef, 0x10, DefaultValue::Ascii(&[b"T"]),
+     "Reference for direction of image"),
+    (GPSImgDirection, 0x11, DefaultValue::None,
+     "Direction of image"),
+    (GPSMapDatum, 0x12, DefaultValue::None,
+     "Geodetic survey data used"),
+    (GPSDestLatitudeRef, 0x13, DefaultValue::None,
+     "Reference for latitude of destination"),
+    (GPSDestLatitude, 0x14, DefaultValue::None,
+     "Latitude of destination"),
+    (GPSDestLongitudeRef, 0x15, DefaultValue::None,
+     "Reference for longitude of destination"),
+    (GPSDestLongitude, 0x16, DefaultValue::None,
+     "Longitude of destination"),
+    (GPSDestBearingRef, 0x17, DefaultValue::Ascii(&[b"T"]),
+     "Reference for bearing of destination"),
+    (GPSDestBearing, 0x18, DefaultValue::None,
+     "Bearing of destination"),
+    (GPSDestDistanceRef, 0x19, DefaultValue::Ascii(&[b"K"]),
+     "Reference for distance to destination"),
+    (GPSDestDistance, 0x1a, DefaultValue::None,
+     "Distance to destination"),
+    (GPSProcessingMethod, 0x1b, DefaultValue::None,
+     "Name of GPS processing method"),
+    (GPSAreaInformation, 0x1c, DefaultValue::None,
+     "Name of GPS area"),
+    (GPSDateStamp, 0x1d, DefaultValue::None,
+     "GPS date"),
+    (GPSDifferential, 0x1e, DefaultValue::None,
+     "GPS differential correction"),
+    (GPSHPositioningError, 0x1f, DefaultValue::None,
+     "Horizontal positioning error"),
 
     // Interoperability attributes [EXIF23 4.6.7 Table 16 and 4.6.8 Table 20].
     |Context::Interop|
 
-    (InteroperabilityIndex, 0x1, "Interoperability identification"),
+    (InteroperabilityIndex, 0x1, DefaultValue::None,
+     "Interoperability identification"),
 );
 
 #[cfg(test)]
@@ -342,4 +501,33 @@ mod tests {
             _ => panic!("failed to match Tag"),
         }
     }
+
+    #[test]
+    fn default_value() {
+        assert_pat!(tag::DateTime.default_value(), None);
+        match tag::BitsPerSample.default_value() {
+            Some(Value::Short(v)) => assert_eq!(v, &[8, 8, 8]),
+            _ => panic!(),
+        }
+        match tag::XResolution.default_value() {
+            Some(Value::Rational(v)) => {
+                assert_eq!(v.len(), 1);
+                assert_eq!(v[0].num, 72);
+                assert_eq!(v[0].denom, 1);
+            },
+            _ => panic!(),
+        }
+        match tag::FileSource.default_value() {
+            Some(Value::Undefined(v)) => assert_eq!(v, &[3]),
+            _ => panic!(),
+        }
+        match tag::GPSAltitudeRef.default_value() {
+            Some(Value::Byte(v)) => assert_eq!(v, &[0]),
+            _ => panic!(),
+        }
+        match tag::GPSSpeedRef.default_value() {
+            Some(Value::Ascii(v)) => assert_eq!(v, &[b"K"]),
+            _ => panic!(),
+        }
+    }
 }
diff --git a/src/value.rs b/src/value.rs
index ec85441..f3f0c04 100644
--- a/src/value.rs
+++ b/src/value.rs
@@ -68,9 +68,43 @@ pub enum Value<'a> {
     Unknown(u16, u32, u32),
 }
 
+// Static default values.
+pub enum DefaultValue {
+    None,
+    Byte(&'static [u8]),
+    Ascii(&'static [&'static [u8]]),
+    Short(&'static [u16]),
+    Rational(&'static [(u32, u32)]),
+    Undefined(&'static [u8]),
+    // Depends on other tags, JPEG markers, etc.
+    ContextDependent,
+    // Unspecified in the Exif standard.
+    Unspecified,
+}
+
+impl<'a> From<&'a DefaultValue> for Option<Value<'a>> {
+    fn from(defval: &DefaultValue) -> Option<Value> {
+        match *defval {
+            DefaultValue::None => None,
+            DefaultValue::Byte(s) => Some(Value::Byte(s.to_vec())),
+            DefaultValue::Ascii(s) => Some(Value::Ascii(s.to_vec())),
+            DefaultValue::Short(s) => Some(Value::Short(s.to_vec())),
+            DefaultValue::Rational(s) => Some(Value::Rational(
+                s.iter().map(|&t| tuple2rational(t)).collect())),
+            DefaultValue::Undefined(s) => Some(Value::Undefined(s)),
+            DefaultValue::ContextDependent => None,
+            DefaultValue::Unspecified => None,
+        }
+    }
+}
+
 /// An unsigned rational number, which is a pair of 32-bit unsigned integers.
 pub struct Rational { pub num: u32, pub denom: u32 }
 
+fn tuple2rational(t: (u32, u32)) -> Rational {
+    Rational { num: t.0, denom: t.1 }
+}
+
 impl fmt::Debug for Rational {
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
         write!(f, "Rational({}/{})", self.num, self.denom)