diff --git a/src/exif.ts b/src/exif.ts
index 9d90df7..224631c 100644
--- a/src/exif.ts
+++ b/src/exif.ts
@@ -1,7 +1,7 @@
 // EXIF in an original's bytes, read with exifreader, which reads it from JPEG,
 // HEIC/HEIF, AVIF, PNG, WebP and the other image formats it supports.
-// `backup-metadata --exif` records every EXIF tag it finds; `Photo.exif()`
-// returns the common fields picked from them here.
+// `backup-metadata --exif` records every EXIF tag it finds except the
+// thumbnail's; `Photo.exif()` returns the common fields picked from them here.
 
 import ExifReader, { type ExpandedTags } from "exifreader";
 
@@ -9,7 +9,11 @@ import ExifReader, { type ExpandedTags } from "exifreader";
 // them (`gps`), and where the EXIF block lies in `bytes` (`metadataRange`).
 // Undefined when exifreader cannot read the file at all, such as a video. An
 // EXIF block it finds but reads no tag from comes back as an empty `exif`.
-// Each tag's `computed` holds its value as a plain string or number.
+// `exif` holds every tag except the thumbnail's; a tag exifreader has no name
+// for is keyed `undefined-<tag number>`. Each tag's `computed` holds its value
+// as a string or number, or as an array of them for a tag with several values,
+// such as `GPSLatitude`'s `[40, 26, 46]`. A fraction with a zero denominator
+// computes to null.
 export const readExifTags = (bytes: Uint8Array): ExpandedTags | undefined => {
     try {
         return ExifReader.loadView(
@@ -18,6 +22,7 @@ export const readExifTags = (bytes: Uint8Array): ExpandedTags | undefined => {
                 expanded: true,
                 computed: true,
                 includeOffsets: true,
+                includeUnknown: true,
                 includeTags: { exif: true, gps: true },
             },
         );
@@ -77,6 +82,7 @@ export const readPhotoExif = (bytes: Uint8Array): PhotoExif => {
     const tags = readExifTags(bytes);
     const exif = tags?.exif;
     const gps = tags?.gps;
+    const altitude = asNumber(exif?.GPSAltitude?.computed);
     const fields: PhotoExif = {
         make: asString(exif?.Make?.computed),
         model: asString(exif?.Model?.computed),
@@ -91,7 +97,11 @@ export const readPhotoExif = (bytes: Uint8Array): PhotoExif => {
         orientation: asNumber(exif?.Orientation?.computed),
         gpsLatitude: asNumber(gps?.Latitude),
         gpsLongitude: asNumber(gps?.Longitude),
-        gpsAltitude: asNumber(gps?.Altitude),
+        // A GPSAltitudeRef of 1 means the altitude is below sea level.
+        gpsAltitude:
+            altitude !== undefined && exif?.GPSAltitudeRef?.value === 1
+                ? -altitude
+                : altitude,
     };
     // Leave out what the file lacks, so a missing field is absent rather than
     // present and undefined.
diff --git a/test/cli/metadata-exif.test.ts b/test/cli/metadata-exif.test.ts
index 2deda57..2bdd533 100644
--- a/test/cli/metadata-exif.test.ts
+++ b/test/cli/metadata-exif.test.ts
@@ -46,6 +46,23 @@ const TIFF_UNSET_DATE = [
     ...new TextEncoder().encode("0000:00:00 00:00:00\0"), // at 56
 ];
 
+// A big-endian TIFF block holding a GPSAltitude of 12.5 m and no
+// GPSAltitudeRef.
+const TIFF_ALTITUDE_WITHOUT_REF = [
+    ...[0x4d, 0x4d, 0x00, 0x2a, 0x00, 0x00, 0x00, 0x08], // the first IFD at 8
+    // The first IFD, at 8: one entry, then no next IFD.
+    ...[0x00, 0x01],
+    // The GPS IFD's offset (0x8825), LONG, 26.
+    ...[0x88, 0x25, 0x00, 0x04, 0x00, 0x00, 0x00, 0x01, 0x00, 0x00, 0x00, 0x1a],
+    ...[0x00, 0x00, 0x00, 0x00],
+    // The GPS IFD, at 26: one entry, then no next IFD.
+    ...[0x00, 0x01],
+    // GPSAltitude (0x0006), one RATIONAL at 44.
+    ...[0x00, 0x06, 0x00, 0x05, 0x00, 0x00, 0x00, 0x01, 0x00, 0x00, 0x00, 0x2c],
+    ...[0x00, 0x00, 0x00, 0x00],
+    ...[0x00, 0x00, 0x00, 0x19, 0x00, 0x00, 0x00, 0x02], // 25/2, at 44
+];
+
 // An APP1 segment whose length field matches its data.
 const app1 = (data: number[]): number[] => {
     const len = data.length + 2;
@@ -70,6 +87,13 @@ describe("readPhotoExif", () => {
         });
     });
 
+    it("reads a GPSAltitude without GPSAltitudeRef as above sea level", () => {
+        const data = [...EXIF_HEADER, ...TIFF_ALTITUDE_WITHOUT_REF];
+        expect(readPhotoExif(bytes(SOI, app1(data), SOS))).toStrictEqual({
+            gpsAltitude: 12.5,
+        });
+    });
+
     it.each([
         [
             "a file that is not an image",