diff --git a/xrspatial/reproject/__init__.py b/xrspatial/reproject/__init__.py
index 8519dcf1..828e47eb 100644
--- a/xrspatial/reproject/__init__.py
+++ b/xrspatial/reproject/__init__.py
@@ -687,15 +687,25 @@ def reproject(
 
     ydim, xdim = _find_spatial_dims(raster)
     # Carry input attrs forward so units, long_name, scale_factor, etc.
-    # survive the transform. Pop attrs that are stale after reprojection:
-    # the affine `transform` and grid `res` describe the old grid, and
-    # `crs_wkt` would duplicate (or contradict) the canonical `crs` we re-emit.
+    # survive the transform. Re-emit `transform` and `res` for the new
+    # output grid (rasterio 6-tuple convention). Drop `crs_wkt` since it
+    # would duplicate (or contradict) the canonical `crs` we re-emit.
+    out_left, _out_bottom, _out_right, out_top = grid['bounds']
+    out_res_x = grid['res_x']
+    out_res_y = grid['res_y']
     out_attrs = {**raster.attrs}
-    out_attrs.pop('transform', None)
     out_attrs.pop('crs_wkt', None)
-    out_attrs.pop('res', None)
     out_attrs['crs'] = tgt_wkt
     out_attrs['nodata'] = nd
+    out_attrs['res'] = (out_res_x, out_res_y)
+    out_attrs['transform'] = (
+        out_res_x, 0.0, out_left, 0.0, -out_res_y, out_top,
+    )
+    # If the input used `_FillValue`, propagate the resolved nodata to
+    # both keys so downstream consumers reading either find a consistent
+    # value. If `_FillValue` was absent, leave it absent.
+    if '_FillValue' in raster.attrs:
+        out_attrs['_FillValue'] = nd
     if tgt_vertical_crs is not None:
         out_attrs['vertical_crs'] = tgt_vertical_crs
 
@@ -1479,7 +1489,8 @@ def merge(
                 "Ensure all rasters have CRS metadata."
             )
         sb = _source_bounds(r)
-        ss = (r.sizes[r.dims[-2]], r.sizes[r.dims[-1]])
+        r_ydim, r_xdim = _find_spatial_dims(r)
+        ss = (r.sizes[r_ydim], r.sizes[r_xdim])
         yd = _is_y_descending(r)
         # Per-raster input nodata sentinel. Detected independently of the
         # user-supplied output nodata so that mixed-sentinel inputs are
@@ -1552,8 +1563,7 @@ def merge(
         )
 
     y_coords, x_coords = _make_output_coords(out_bounds, out_shape)
-    ydim = rasters[0].dims[-2]
-    xdim = rasters[0].dims[-1]
+    ydim, xdim = _find_spatial_dims(rasters[0])
 
     out_coords = {ydim: y_coords, xdim: x_coords}
     # Carry forward non-spatial coords from the first raster (e.g. scalar
@@ -1567,14 +1577,24 @@ def merge(
         out_coords[cname] = cval
 
     # Carry the first raster's attrs forward (matches the default
-    # strategy='first'). Drop attrs describing the old grid: `transform`,
-    # `res`, and the duplicate `crs_wkt` are no longer accurate.
+    # strategy='first'). Drop the duplicate `crs_wkt` and re-emit
+    # `transform` and `res` for the new output grid (rasterio 6-tuple).
+    out_left, _out_bottom, _out_right, out_top = grid['bounds']
+    out_res_x = grid['res_x']
+    out_res_y = grid['res_y']
     out_attrs = {**rasters[0].attrs}
-    out_attrs.pop('transform', None)
     out_attrs.pop('crs_wkt', None)
-    out_attrs.pop('res', None)
     out_attrs['crs'] = tgt_wkt
     out_attrs['nodata'] = nd
+    out_attrs['res'] = (out_res_x, out_res_y)
+    out_attrs['transform'] = (
+        out_res_x, 0.0, out_left, 0.0, -out_res_y, out_top,
+    )
+    # If the first raster used `_FillValue`, propagate the resolved
+    # nodata to both keys for consistent round-trip. Leave absent
+    # otherwise.
+    if '_FillValue' in rasters[0].attrs:
+        out_attrs['_FillValue'] = nd
 
     result = xr.DataArray(
         result_data,
diff --git a/xrspatial/tests/test_reproject.py b/xrspatial/tests/test_reproject.py
index d7e4b504..5c950845 100644
--- a/xrspatial/tests/test_reproject.py
+++ b/xrspatial/tests/test_reproject.py
@@ -2121,19 +2121,20 @@ def test_reproject_preserves_long_name(self):
         result = reproject(raster, 'EPSG:4326', resolution=0.25)
         assert result.attrs.get('long_name') == 'elevation'
 
-    def test_reproject_drops_stale_transform(self):
+    def test_reproject_replaces_stale_transform(self):
         from xrspatial.reproject import reproject
-        raster = self._raster_with_attrs(
-            {'transform': (1.0, 0.0, 0.0, 0.0, -1.0, 0.0)}
-        )
+        stale = (1.0, 0.0, 0.0, 0.0, -1.0, 0.0)
+        raster = self._raster_with_attrs({'transform': stale})
         result = reproject(raster, 'EPSG:3857')
-        assert 'transform' not in result.attrs
+        assert 'transform' in result.attrs
+        assert tuple(result.attrs['transform']) != stale
 
-    def test_reproject_drops_stale_res(self):
+    def test_reproject_replaces_stale_res(self):
         from xrspatial.reproject import reproject
         raster = self._raster_with_attrs({'res': (1.0, 1.0)})
         result = reproject(raster, 'EPSG:3857')
-        assert 'res' not in result.attrs
+        assert 'res' in result.attrs
+        assert tuple(result.attrs['res']) != (1.0, 1.0)
 
     def test_reproject_overrides_crs(self):
         from xrspatial.reproject import reproject
@@ -2168,17 +2169,171 @@ def test_merge_preserves_first_raster_attrs(self):
         assert result.attrs.get('units') == 'm'
         assert result.attrs.get('long_name') == 'elev'
 
-    def test_merge_drops_stale_transform(self):
+    def test_merge_replaces_stale_transform(self):
         from xrspatial.reproject import merge
+        stale = (1.0, 0.0, 0.0, 0.0, -1.0, 0.0)
         a = self._raster_with_attrs(
-            {'transform': (1.0, 0.0, 0.0, 0.0, -1.0, 0.0)},
+            {'transform': stale},
             x_range=(-5, 0), y_range=(-5, 5),
         )
         b = self._raster_with_attrs(
             x_range=(0, 5), y_range=(-5, 5),
         )
         result = merge([a, b], resolution=1.0)
-        assert 'transform' not in result.attrs
+        assert 'transform' in result.attrs
+        assert tuple(result.attrs['transform']) != stale
+
+    # Fresh transform/res emission ----------------------------------------
+
+    def test_reproject_emits_fresh_transform(self):
+        from xrspatial.reproject import reproject
+        stale = (1.0, 0.0, 0.0, 0.0, -1.0, 10.0)
+        raster = self._raster_with_attrs({'transform': stale})
+        result = reproject(raster, 'EPSG:3857', resolution=50000.0)
+        t = result.attrs['transform']
+        assert len(t) == 6
+        # Output is in EPSG:3857 so transform values cannot match the stale
+        # geographic-degree input.
+        assert tuple(t) != stale
+        # transform[0] is res_x, transform[4] is -res_y, transform[2] is
+        # left edge, transform[5] is top edge.
+        res_x, res_y = result.attrs['res']
+        assert t[0] == res_x
+        assert t[4] == -res_y
+        # Top edge: y coord of first row plus half a pixel.
+        y0 = float(result.coords['y'].values[0])
+        assert t[5] == pytest.approx(y0 + res_y / 2)
+        # Left edge: x coord of first col minus half a pixel.
+        x0 = float(result.coords['x'].values[0])
+        assert t[2] == pytest.approx(x0 - res_x / 2)
+
+    def test_reproject_emits_fresh_res(self):
+        from xrspatial.reproject import reproject
+        raster = self._raster_with_attrs({'res': (1.0, 1.0)})
+        # Use an explicit resolution very different from input.
+        result = reproject(raster, 'EPSG:4326', resolution=0.25)
+        assert 'res' in result.attrs
+        res_x, res_y = result.attrs['res']
+        # Pixel size derived from output coords must match.
+        x = result.coords['x'].values
+        y = result.coords['y'].values
+        actual_res_x = float(abs(x[1] - x[0]))
+        actual_res_y = float(abs(y[1] - y[0]))
+        assert res_x == pytest.approx(actual_res_x)
+        assert res_y == pytest.approx(actual_res_y)
+
+    def test_reproject_no_input_transform_still_emits_one(self):
+        from xrspatial.reproject import reproject
+        raster = self._raster_with_attrs()
+        assert 'transform' not in raster.attrs
+        result = reproject(raster, 'EPSG:4326', resolution=0.25)
+        assert 'transform' in result.attrs
+        assert 'res' in result.attrs
+        assert len(result.attrs['transform']) == 6
+
+    def test_merge_emits_fresh_transform_and_res(self):
+        from xrspatial.reproject import merge
+        a = self._raster_with_attrs(
+            x_range=(-5, 0), y_range=(-5, 5),
+        )
+        b = self._raster_with_attrs(
+            x_range=(0, 5), y_range=(-5, 5),
+        )
+        result = merge([a, b], resolution=1.0)
+        assert 'transform' in result.attrs
+        assert 'res' in result.attrs
+        t = result.attrs['transform']
+        assert len(t) == 6
+        res_x, res_y = result.attrs['res']
+        assert t[0] == res_x
+        assert t[4] == -res_y
+        y0 = float(result.coords['y'].values[0])
+        x0 = float(result.coords['x'].values[0])
+        assert t[5] == pytest.approx(y0 + res_y / 2)
+        assert t[2] == pytest.approx(x0 - res_x / 2)
+
+    def test_merge_finds_spatial_dims_with_lat_lon(self):
+        from xrspatial.reproject import merge
+        a_data = np.ones((8, 8), dtype=np.float64)
+        b_data = np.ones((8, 8), dtype=np.float64) * 2
+        attrs = {'crs': 'EPSG:4326', 'nodata': np.nan}
+        a = xr.DataArray(
+            a_data, dims=['lat', 'lon'],
+            coords={
+                'lat': np.linspace(5, -5, 8),
+                'lon': np.linspace(-5, 0, 8),
+            },
+            name='a', attrs=attrs,
+        )
+        b = xr.DataArray(
+            b_data, dims=['lat', 'lon'],
+            coords={
+                'lat': np.linspace(5, -5, 8),
+                'lon': np.linspace(0, 5, 8),
+            },
+            name='b', attrs=attrs,
+        )
+        result = merge([a, b], resolution=1.0)
+        assert result.dims == ('lat', 'lon')
+        assert 'lat' in result.coords
+        assert 'lon' in result.coords
+
+    # _FillValue propagation -----------------------------------------------
+
+    def test_reproject_propagates_fill_value(self):
+        from xrspatial.reproject import reproject
+        # Build a raster with _FillValue set and no nodata key.
+        data = np.ones((8, 8), dtype=np.float64)
+        attrs = {'crs': 'EPSG:4326', '_FillValue': -9999}
+        raster = xr.DataArray(
+            data, dims=['y', 'x'],
+            coords={
+                'y': np.linspace(1, -1, 8),
+                'x': np.linspace(-1, 1, 8),
+            },
+            attrs=attrs,
+        )
+        result = reproject(raster, 'EPSG:4326', resolution=0.25)
+        assert '_FillValue' in result.attrs
+        assert 'nodata' in result.attrs
+        assert result.attrs['_FillValue'] == result.attrs['nodata']
+        assert result.attrs['_FillValue'] == -9999
+
+    def test_reproject_omits_fill_value_when_input_omits(self):
+        from xrspatial.reproject import reproject
+        raster = self._raster_with_attrs()
+        assert '_FillValue' not in raster.attrs
+        result = reproject(raster, 'EPSG:4326', resolution=0.25)
+        assert '_FillValue' not in result.attrs
+        assert 'nodata' in result.attrs
+
+    def test_merge_propagates_fill_value(self):
+        from xrspatial.reproject import merge
+        a_data = np.ones((8, 8), dtype=np.float64)
+        b_data = np.ones((8, 8), dtype=np.float64) * 2
+        attrs_a = {'crs': 'EPSG:4326', '_FillValue': -9999}
+        attrs_b = {'crs': 'EPSG:4326', '_FillValue': -9999}
+        a = xr.DataArray(
+            a_data, dims=['y', 'x'],
+            coords={
+                'y': np.linspace(5, -5, 8),
+                'x': np.linspace(-5, 0, 8),
+            },
+            name='a', attrs=attrs_a,
+        )
+        b = xr.DataArray(
+            b_data, dims=['y', 'x'],
+            coords={
+                'y': np.linspace(5, -5, 8),
+                'x': np.linspace(0, 5, 8),
+            },
+            name='b', attrs=attrs_b,
+        )
+        result = merge([a, b], resolution=1.0)
+        assert '_FillValue' in result.attrs
+        assert 'nodata' in result.attrs
+        assert result.attrs['_FillValue'] == result.attrs['nodata']
+        assert result.attrs['_FillValue'] == -9999
 
     def test_merge_name_falls_back_to_first_raster(self):
         from xrspatial.reproject import merge