diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 7198b49..236d26a 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -18,5 +18,8 @@ jobs:
       - name: Check generated files are up to date
         run: pixi run check
 
+      - name: Run tests
+        run: pixi run test
+
       # Use ruff-action so we get annotations in the Github UI
       - uses: astral-sh/ruff-action@v4.1.0
diff --git a/pixi.lock b/pixi.lock
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       - conda: https://conda.anaconda.org/conda-forge/noarch/decorator-5.2.1-pyhd8ed1ab_0.conda
+      - conda: https://conda.anaconda.org/conda-forge/noarch/exceptiongroup-1.3.1-pyhd8ed1ab_0.conda
       - conda: https://conda.anaconda.org/conda-forge/noarch/executing-2.2.1-pyhd8ed1ab_0.conda
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@@ -475,6 +500,7 @@ environments:
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       - conda: https://conda.anaconda.org/conda-forge/win-64/imagecodecs-2026.6.26-py314h37ae81f_0.conda
+      - conda: https://conda.anaconda.org/conda-forge/noarch/iniconfig-2.3.0-pyhd8ed1ab_0.conda
       - conda: https://conda.anaconda.org/conda-forge/noarch/ipykernel-7.1.0-pyh6dadd2b_0.conda
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@@ -537,12 +563,14 @@ environments:
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       - conda: https://conda.anaconda.org/conda-forge/noarch/platformdirs-4.5.1-pyhcf101f3_0.conda
+      - conda: https://conda.anaconda.org/conda-forge/noarch/pluggy-1.6.0-pyhf9edf01_1.conda
       - conda: https://conda.anaconda.org/conda-forge/win-64/proj-9.7.1-h7b1ce8f_0.conda
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       - conda: https://conda.anaconda.org/conda-forge/noarch/pyparsing-3.3.2-pyhcf101f3_0.conda
+      - conda: https://conda.anaconda.org/conda-forge/noarch/pytest-9.1.1-pyhc364b38_2.conda
       - conda: https://conda.anaconda.org/conda-forge/win-64/python-3.14.2-h4b44e0e_100_cp314.conda
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       - conda: https://conda.anaconda.org/conda-forge/noarch/python-gil-3.14.2-h4df99d1_100.conda
@@ -561,6 +589,7 @@ environments:
       - conda: https://conda.anaconda.org/conda-forge/win-64/tbb-2022.3.0-h3155e25_2.conda
       - conda: https://conda.anaconda.org/conda-forge/noarch/tifffile-2026.7.31-pyhd8ed1ab_0.conda
       - conda: https://conda.anaconda.org/conda-forge/win-64/tk-8.6.13-h2c6b04d_3.conda
+      - conda: https://conda.anaconda.org/conda-forge/noarch/tomli-2.4.1-pyhcf101f3_0.conda
       - conda: https://conda.anaconda.org/conda-forge/win-64/tornado-6.5.4-py314h5a2d7ad_0.conda
       - conda: https://conda.anaconda.org/conda-forge/noarch/traitlets-5.14.3-pyhd8ed1ab_1.conda
       - conda: https://conda.anaconda.org/conda-forge/noarch/typing_extensions-4.15.0-pyhcf101f3_0.conda
@@ -579,7 +608,9 @@ environments:
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+      - pypi: https://files.pythonhosted.org/packages/9f/cc/e106fcee3a8cd98d57c8842ba4ff2839fea6ca736970f0899b3162298fc9/async_tiff-0.7.2-cp311-abi3-win_amd64.whl
       - pypi: https://files.pythonhosted.org/packages/f1/c3/cb8db47ed1a28b43d498e91b629d69886778ad5d9e91dcb953d411052728/morecantile-7.0.2-py3-none-any.whl
+      - pypi: https://files.pythonhosted.org/packages/bb/69/96feeac84ce0b871567225c78515f3b557c023e72ed9b4f1833f3662bd6b/obspec-0.1.0-py3-none-any.whl
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@@ -736,6 +767,34 @@ packages:
   - pkg:pypi/asttokens?source=hash-mapping
   size: 28797
   timestamp: 1763410017955
+- pypi: https://files.pythonhosted.org/packages/3e/8f/c8a7c48fd966e13ee7390984d43318f71f0eab76889108b3a9621b6ef62a/async_tiff-0.7.2-cp311-abi3-manylinux_2_28_x86_64.whl
+  name: async-tiff
+  version: 0.7.2
+  sha256: 84ca597d8a6354d7b1ae46d2c41432e0dad36ffd4ad54f45aa29d11857506e9b
+  requires_dist:
+  - obspec>=0.1.0
+  requires_python: '>=3.10'
+- pypi: https://files.pythonhosted.org/packages/6b/8b/1591e69991987b43ccd7faecb843bd601dfb08a35395770c1562aaca73da/async_tiff-0.7.2-cp311-abi3-macosx_10_12_x86_64.whl
+  name: async-tiff
+  version: 0.7.2
+  sha256: 028e9d629dbe8d830581d8e46b92081a656953db020b424615af3c6e51ed9249
+  requires_dist:
+  - obspec>=0.1.0
+  requires_python: '>=3.10'
+- pypi: https://files.pythonhosted.org/packages/9f/cc/e106fcee3a8cd98d57c8842ba4ff2839fea6ca736970f0899b3162298fc9/async_tiff-0.7.2-cp311-abi3-win_amd64.whl
+  name: async-tiff
+  version: 0.7.2
+  sha256: 296647adac8b05ae1bacc69feea2e42ce17220d34df1bc419be0daa94364ebd5
+  requires_dist:
+  - obspec>=0.1.0
+  requires_python: '>=3.10'
+- pypi: https://files.pythonhosted.org/packages/c5/58/fc2451333d389da41e8505a8edcc711fcacfb8f038c67a1629d7e87fbdc0/async_tiff-0.7.2-cp311-abi3-macosx_11_0_arm64.whl
+  name: async-tiff
+  version: 0.7.2
+  sha256: 97d77fcad25824321645c3b83da060b1ece827f30e76fe64db74b4df97e5da1d
+  requires_dist:
+  - obspec>=0.1.0
+  requires_python: '>=3.10'
 - conda: https://conda.anaconda.org/conda-forge/noarch/attrs-25.4.0-pyhcf101f3_1.conda
   sha256: c13d5e42d187b1d0255f591b7ce91201d4ed8a5370f0d986707a802c20c9d32f
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@@ -1252,6 +1311,17 @@ packages:
   - pkg:pypi/decorator?source=hash-mapping
   size: 14129
   timestamp: 1740385067843
+- conda: https://conda.anaconda.org/conda-forge/noarch/exceptiongroup-1.3.1-pyhd8ed1ab_0.conda
+  sha256: ee6cf346d017d954255bbcbdb424cddea4d14e4ed7e9813e429db1d795d01144
+  md5: 8e662bd460bda79b1ea39194e3c4c9ab
+  depends:
+  - python >=3.10
+  - typing_extensions >=4.6.0
+  license: MIT and PSF-2.0
+  purls:
+  - pkg:pypi/exceptiongroup?source=hash-mapping
+  size: 21333
+  timestamp: 1763918099466
 - conda: https://conda.anaconda.org/conda-forge/noarch/executing-2.2.1-pyhd8ed1ab_0.conda
   sha256: 210c8165a58fdbf16e626aac93cc4c14dbd551a01d1516be5ecad795d2422cad
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@@ -1691,6 +1761,17 @@ packages:
   - pkg:pypi/imagecodecs?source=hash-mapping
   size: 2008465
   timestamp: 1782686195641
+- conda: https://conda.anaconda.org/conda-forge/noarch/iniconfig-2.3.0-pyhd8ed1ab_0.conda
+  sha256: e1a9e3b1c8fe62dc3932a616c284b5d8cbe3124bbfbedcf4ce5c828cb166ee19
+  md5: 9614359868482abba1bd15ce465e3c42
+  depends:
+  - python >=3.10
+  license: MIT
+  license_family: MIT
+  purls:
+  - pkg:pypi/iniconfig?source=hash-mapping
+  size: 13387
+  timestamp: 1760831448842
 - conda: https://conda.anaconda.org/conda-forge/noarch/ipykernel-7.1.0-pyh5552912_0.conda
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@@ -4871,6 +4952,13 @@ packages:
   - pkg:pypi/numpy?source=hash-mapping
   size: 7306379
   timestamp: 1768085588568
+- pypi: https://files.pythonhosted.org/packages/bb/69/96feeac84ce0b871567225c78515f3b557c023e72ed9b4f1833f3662bd6b/obspec-0.1.0-py3-none-any.whl
+  name: obspec
+  version: 0.1.0
+  sha256: 307f0fa2c2998b324ecf0eed6a2a89049a4c40c9b1fa2b5e1af28f0ee72136b3
+  requires_dist:
+  - typing-extensions ; python_full_version < '3.12'
+  requires_python: '>=3.9'
 - conda: https://conda.anaconda.org/conda-forge/linux-64/openjpeg-2.5.4-h55fea9a_0.conda
   sha256: 3900f9f2dbbf4129cf3ad6acf4e4b6f7101390b53843591c53b00f034343bc4d
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@@ -5124,6 +5212,18 @@ packages:
   - pkg:pypi/platformdirs?source=hash-mapping
   size: 23922
   timestamp: 1764950726246
+- conda: https://conda.anaconda.org/conda-forge/noarch/pluggy-1.6.0-pyhf9edf01_1.conda
+  sha256: e14aafa63efa0528ca99ba568eaf506eb55a0371d12e6250aaaa61718d2eb62e
+  md5: d7585b6550ad04c8c5e21097ada2888e
+  depends:
+  - python >=3.9
+  - python
+  license: MIT
+  license_family: MIT
+  purls:
+  - pkg:pypi/pluggy?source=hash-mapping
+  size: 25877
+  timestamp: 1764896838868
 - conda: https://conda.anaconda.org/conda-forge/linux-64/proj-9.7.1-h99ae125_0.conda
   sha256: 551cd2b779902ff88cb945cd69af9978561347a17023403b64f476a5a82b70c5
   md5: 8bbc19a6e87fbe8b97796e9a42a47a30
@@ -5376,6 +5476,27 @@ packages:
   requires_dist:
   - certifi
   requires_python: '>=3.11'
+- conda: https://conda.anaconda.org/conda-forge/noarch/pytest-9.1.1-pyhc364b38_2.conda
+  sha256: 430051d80765207a7d782b2b188230ba1489d35c6e75fd9903f76cb9fda4af16
+  md5: 64c98a12c4e23eb238bf66bbecafdf3c
+  depends:
+  - colorama
+  - pygments >=2.7.2
+  - python >=3.10
+  - iniconfig >=1.0.1
+  - packaging >=22
+  - pluggy >=1.5,<2
+  - tomli >=1
+  - exceptiongroup >=1
+  - python
+  constrains:
+  - pytest-faulthandler >=2
+  license: MIT
+  license_family: MIT
+  purls:
+  - pkg:pypi/pytest?source=hash-mapping
+  size: 306724
+  timestamp: 1782127176429
 - conda: https://conda.anaconda.org/conda-forge/linux-64/python-3.14.2-h32b2ec7_100_cp314.conda
   build_number: 100
   sha256: a120fb2da4e4d51dd32918c149b04a08815fd2bd52099dad1334647984bb07f1
@@ -6076,6 +6197,18 @@ packages:
   purls: []
   size: 3472313
   timestamp: 1763055164278
+- conda: https://conda.anaconda.org/conda-forge/noarch/tomli-2.4.1-pyhcf101f3_0.conda
+  sha256: 91cafdb64268e43e0e10d30bd1bef5af392e69f00edd34dfaf909f69ab2da6bd
+  md5: b5325cf06a000c5b14970462ff5e4d58
+  depends:
+  - python >=3.10
+  - python
+  license: MIT
+  license_family: MIT
+  purls:
+  - pkg:pypi/tomli?source=hash-mapping
+  size: 21561
+  timestamp: 1774492402955
 - conda: https://conda.anaconda.org/conda-forge/linux-64/tornado-6.5.3-py314h5bd0f2a_0.conda
   sha256: b8f9f9ae508d79c9c697eb01b6a8d2ed4bc1899370f44aa6497c8abbd15988ea
   md5: e35f08043f54d26a1be93fdbf90d30c3
diff --git a/pixi.toml b/pixi.toml
index f770f72..6f22a6e 100644
--- a/pixi.toml
+++ b/pixi.toml
@@ -10,6 +10,7 @@ generate = "python generate_fixtures.py"
 info = "bash scripts/info.sh"
 check = "bash scripts/check.sh"
 generate-npy = "python scripts/generate_npy.py"
+test = "pytest tests"
 
 [dependencies]
 gdal = ">=3.12.1,<4"
@@ -19,6 +20,8 @@ python = ">=3.12"
 rasterio = ">=1.5.0,<2"
 tifffile = ">=2024.1.30"
 affine = ">=2.4.0,<3"
+pytest = ">=8"
 
 [pypi-dependencies]
 rio-cogeo = ">=7.0.1, <8"
+async-tiff = ">=0.7.2, <0.8"
diff --git a/real_data/source-coop-dataforcanada/O2308000_7586000_cog.tif b/real_data/source-coop-dataforcanada/O2308000_7586000_cog.tif
new file mode 100644
index 0000000..11adc1f
Binary files /dev/null and b/real_data/source-coop-dataforcanada/O2308000_7586000_cog.tif differ
diff --git a/real_data/source-coop-dataforcanada/O2308000_7586000_cog_info.md b/real_data/source-coop-dataforcanada/O2308000_7586000_cog_info.md
new file mode 100644
index 0000000..cf3e88c
--- /dev/null
+++ b/real_data/source-coop-dataforcanada/O2308000_7586000_cog_info.md
@@ -0,0 +1,61 @@
+```
+Driver: GTiff
+File: real_data/source-coop-dataforcanada/O2308000_7586000_cog.tif
+COG: True
+Compression: LZW
+ColorSpace: None
+
+Profile
+    Width:            128
+    Height:           128
+    Bands:            3
+    Tiled:            False
+    Dtype:            uint8
+    NoData:           None
+    Alpha Band:       False
+    Internal Mask:    False
+    Interleave:       PIXEL
+    ColorMap:         False
+    ColorInterp:      ('red', 'green', 'blue')
+    Scales:           (1.0, 1.0, 1.0)
+    Offsets:          (0.0, 0.0, 0.0)
+
+Geo
+    Crs:              EPSG:2953
+    Origin:           (2307999.0, 7588002.0)
+    Resolution:       (0.1, -0.1)
+    BoundingBox:      (2307999.0, 7587989.2, 2308011.8, 7588002.0)
+    MinZoom:          20
+    MaxZoom:          20
+
+Image Metadata
+    DataType: Generic
+    AREA_OR_POINT: Area
+
+Image Structure
+    LAYOUT: COG
+    COMPRESSION: LZW
+    INTERLEAVE: PIXEL
+
+Band 1
+    ColorInterp: red
+    Metadata:
+        BandName: Band_1
+        RepresentationType: ATHEMATIC
+
+Band 2
+    ColorInterp: green
+    Metadata:
+        BandName: Band_2
+        RepresentationType: ATHEMATIC
+
+Band 3
+    ColorInterp: blue
+    Metadata:
+        BandName: Band_3
+        RepresentationType: ATHEMATIC
+
+IFD
+    Id      Size           BlockSize     Decimation
+    0       128x128        128x128       0
+```
diff --git a/real_data/source-coop-dataforcanada/README.md b/real_data/source-coop-dataforcanada/README.md
new file mode 100644
index 0000000..d5f9d18
--- /dev/null
+++ b/real_data/source-coop-dataforcanada/README.md
@@ -0,0 +1,54 @@
+Includes data from
+
+https://source.coop/dataforcanada
+
+## User-defined Oblique Stereographic CRS
+
+New Brunswick Stereographic, equivalent to EPSG:2953, but stored as a
+user-defined projected CRS (`ProjectedCSTypeGeoKey = 32767`) with
+`ProjCoordTransGeoKey = 16` (`CT_ObliqueStereographic`). The projection origin
+is stored in `ProjNatOriginLatGeoKey`, `ProjNatOriginLongGeoKey` and
+`ProjScaleAtNatOriginGeoKey`, not the `ProjCenter*` keys.
+
+See https://github.com/source-cooperative/cog-viewer/issues/40.
+
+`listgeo -no_norm O2308000_7586000_cog.tif`:
+
+```
+   Keyed_Information:
+      GTModelTypeGeoKey (Short,1): ModelTypeProjected
+      GTRasterTypeGeoKey (Short,1): RasterPixelIsArea
+      GTCitationGeoKey (Ascii,42): "NAD83(CSRS) / New Brunswick Stereographic"
+      GeographicTypeGeoKey (Short,1): User-Defined
+      GeogCitationGeoKey (Ascii,43): "GCS Name = NAD83(CSRS)|Primem = Greenwich|"
+      GeogGeodeticDatumGeoKey (Short,1): Code-6140 (NAD83 Canadian Spatial Reference System)
+      GeogAngularUnitsGeoKey (Short,1): Angular_Degree
+      GeogSemiMajorAxisGeoKey (Double,1): 6378137
+      GeogInvFlatteningGeoKey (Double,1): 298.257222101
+      GeogPrimeMeridianLongGeoKey (Double,1): 0
+      ProjectedCSTypeGeoKey (Short,1): User-Defined
+      ProjectionGeoKey (Short,1): User-Defined
+      ProjCoordTransGeoKey (Short,1): CT_ObliqueStereographic
+      ProjLinearUnitsGeoKey (Short,1): Linear_Meter
+      ProjNatOriginLongGeoKey (Double,1): -66.5
+      ProjNatOriginLatGeoKey (Double,1): 46.5
+      ProjFalseEastingGeoKey (Double,1): 2500000
+      ProjFalseNorthingGeoKey (Double,1): 7500000
+      ProjScaleAtNatOriginGeoKey (Double,1): 0.999912
+      End_Of_Keys.
+```
+
+GDAL identifies the source CRS as EPSG:2953 by name when reading, so a plain
+`gdal_translate` writes `ProjectedCSTypeGeoKey = 2953` instead. Assigning the
+same WKT without the `PROJCS` and `GEOGCS` authority codes makes GDAL write the
+same user-defined geo keys as the source file:
+
+```bash
+pixi run gdal_translate \
+  -srcwin 0 0 128 128 \
+  -a_srs 'PROJCS["NAD83(CSRS) / New Brunswick Stereographic",GEOGCS["NAD83(CSRS)",DATUM["NAD83_Canadian_Spatial_Reference_System",SPHEROID["GRS 1980",6378137,298.257222101,AUTHORITY["EPSG","7019"]],AUTHORITY["EPSG","6140"]],PRIMEM["Greenwich",0,AUTHORITY["EPSG","8901"]],UNIT["degree",0.0174532925199433,AUTHORITY["EPSG","9122"]]],PROJECTION["Oblique_Stereographic"],PARAMETER["latitude_of_origin",46.5],PARAMETER["central_meridian",-66.5],PARAMETER["scale_factor",0.999912],PARAMETER["false_easting",2500000],PARAMETER["false_northing",7500000],UNIT["metre",1,AUTHORITY["EPSG","9001"]]]' \
+  -of COG \
+  -co BLOCKSIZE=128 \
+  /vsicurl/https://data.source.coop/dataforcanada/test/O2308000_7586000_cog.tif \
+  O2308000_7586000_cog.tif
+```
diff --git a/tests/test_real_data.py b/tests/test_real_data.py
new file mode 100644
index 0000000..daf30fb
--- /dev/null
+++ b/tests/test_real_data.py
@@ -0,0 +1,58 @@
+"""Check the raw GeoKeys of real_data fixtures.
+
+Some fixtures exist to exercise a specific GeoKey layout, which regenerating
+them with GDAL can silently change: GDAL may identify a user-defined CRS as an
+EPSG code and write only that code. Reading the raw keys with async-tiff
+catches that.
+"""
+
+import asyncio
+from pathlib import Path
+from typing import Any
+
+from async_tiff import TIFF
+from async_tiff.store import LocalStore
+
+REPO_ROOT = Path(__file__).parent.parent
+
+
+def read_geo_keys(path: str) -> dict[str, Any]:
+    """Read the GeoKeys of the first IFD of a TIFF, relative to the repo root."""
+
+    async def _read() -> dict[str, Any]:
+        tiff = await TIFF.open(path, store=LocalStore(REPO_ROOT))
+        gkd = tiff.ifds[0].geo_key_directory
+        return {key: gkd[key] for key in gkd.keys()}
+
+    return asyncio.run(_read())
+
+
+def test_dataforcanada_user_defined_oblique_stereographic():
+    """Assert that CRS is stored in custom geo keys, not simply as `EPSG:2953`.
+
+    See real_data/source-coop-dataforcanada/README.md.
+    """
+    geo_keys = read_geo_keys(
+        "real_data/source-coop-dataforcanada/O2308000_7586000_cog.tif"
+    )
+    assert geo_keys == {
+        "model_type": 1,
+        "raster_type": 1,
+        "citation": "NAD83(CSRS) / New Brunswick Stereographic",
+        "geographic_type": 32767,
+        "geog_citation": "GCS Name = NAD83(CSRS)|Primem = Greenwich|",
+        "geog_geodetic_datum": 6140,
+        "geog_angular_units": 9102,
+        "geog_semi_major_axis": 6378137.0,
+        "geog_inv_flattening": 298.257222101,
+        "geog_prime_meridian_long": 0.0,
+        "projected_type": 32767,
+        "projection": 32767,
+        "proj_coord_trans": 16,
+        "proj_linear_units": 9001,
+        "proj_nat_origin_long": -66.5,
+        "proj_nat_origin_lat": 46.5,
+        "proj_false_easting": 2500000.0,
+        "proj_false_northing": 7500000.0,
+        "proj_scale_at_nat_origin": 0.999912,
+    }