From 442b62beb397667cecfefdfc02046d00f9e9d0a6 Mon Sep 17 00:00:00 2001
From: Deepak kudi <deepakkudi23@adsl-172-10-9-116.dsl.sndg02.sbcglobal.net>
Date: Thu, 21 May 2026 01:07:42 +0530
Subject: [PATCH] Validate reproject coordinate spacing

---
 xrspatial/reproject/__init__.py   | 40 ++++++++++++++++++++++++++++++
 xrspatial/tests/test_reproject.py | 41 +++++++++++++++++++++++++++++++
 2 files changed, 81 insertions(+)

diff --git a/xrspatial/reproject/__init__.py b/xrspatial/reproject/__init__.py
index a04f58dc5..dd12eaeac 100644
--- a/xrspatial/reproject/__init__.py
+++ b/xrspatial/reproject/__init__.py
@@ -68,6 +68,8 @@
     'ellipsoidal': 4979,  # WGS 84 (3D, ellipsoidal height)
 }
 
+_SPATIAL_COORD_RTOL = 1e-6
+
 
 def _find_spatial_dims(raster):
     """Find the y and x dimension names, handling multi-band rasters.
@@ -88,6 +90,41 @@ def _find_spatial_dims(raster):
     return dims[-2], dims[-1]
 
 
+def _validate_regular_spatial_coords(raster, context):
+    """Validate that spatial coordinates describe a rectilinear grid."""
+    ydim, xdim = _find_spatial_dims(raster)
+    _validate_regular_axis(raster.coords[ydim].values, 'y', context)
+    _validate_regular_axis(raster.coords[xdim].values, 'x', context)
+
+
+def _validate_regular_axis(values, axis_name, context):
+    values = np.asarray(values, dtype=np.float64)
+    if values.size < 2:
+        return
+
+    diffs = np.diff(values)
+    if not np.all(np.isfinite(diffs)):
+        raise ValueError(
+            f"{context}: {axis_name} coordinates must contain finite values"
+        )
+
+    if not (np.all(diffs > 0) or np.all(diffs < 0)):
+        raise ValueError(
+            f"{context}: {axis_name} coordinates must be strictly monotonic "
+            "(all ascending or all descending)"
+        )
+
+    median_step = float(np.median(diffs))
+    worst_step_deviation = float(np.max(np.abs(diffs - median_step)))
+    tolerance = _SPATIAL_COORD_RTOL * abs(median_step)
+    if worst_step_deviation > tolerance:
+        raise ValueError(
+            f"{context}: {axis_name} coordinates must be regularly spaced; "
+            f"worst step deviation is {worst_step_deviation:g} "
+            f"(median step {median_step:g}, rtol {_SPATIAL_COORD_RTOL:g})"
+        )
+
+
 def _source_bounds(raster):
     """Extract (left, bottom, right, top) from a DataArray's coordinates."""
     ydim, xdim = _find_spatial_dims(raster)
@@ -642,6 +679,8 @@ def reproject(
 
     _validate_resampling(resampling)
 
+    _validate_regular_spatial_coords(raster, "reproject()")
+
     # Reject unknown vertical-datum tokens at the API boundary so we never
     # write None into attrs['vertical_crs'] for typos / unsupported values.
     for _name, _val in (('src_vertical_crs', src_vertical_crs),
@@ -1751,6 +1790,7 @@ def merge(
         # (#2027). Reject 3-D up front so callers get a clear error.
         _validate_raster(r, func_name='merge', name=f'rasters[{i}]',
                          ndim=(2,))
+        _validate_regular_spatial_coords(r, f"merge(): rasters[{i}]")
 
     _validate_grid_params(
         resolution=resolution,
diff --git a/xrspatial/tests/test_reproject.py b/xrspatial/tests/test_reproject.py
index a76efd048..19e397cd1 100644
--- a/xrspatial/tests/test_reproject.py
+++ b/xrspatial/tests/test_reproject.py
@@ -505,6 +505,33 @@ def test_missing_crs_raises(self):
         with pytest.raises(ValueError, match="source CRS"):
             reproject(raster, 'EPSG:3857')
 
+    def test_reproject_rejects_irregular_x_before_crs_resolution(self):
+        from xrspatial.reproject import reproject
+        raster = xr.DataArray(
+            np.zeros((4, 4)), dims=['y', 'x'],
+            coords={'y': [3.0, 2.0, 1.0, 0.0],
+                    'x': [0.0, 1.0, 2.3, 3.0]},
+        )
+        with pytest.raises(
+            ValueError,
+            match=r"reproject\(\): x coordinates.*regular.*worst step deviation",
+        ):
+            reproject(raster, 'EPSG:4326')
+
+    def test_reproject_rejects_non_monotonic_y(self):
+        from xrspatial.reproject import reproject
+        raster = xr.DataArray(
+            np.zeros((4, 4)), dims=['y', 'x'],
+            coords={'y': [3.0, 2.0, 2.5, 0.0],
+                    'x': [0.0, 1.0, 2.0, 3.0]},
+            attrs={'crs': 'EPSG:4326'},
+        )
+        with pytest.raises(
+            ValueError,
+            match=r"reproject\(\): y coordinates must be strictly monotonic",
+        ):
+            reproject(raster, 'EPSG:4326')
+
     def test_non_dataarray_raises(self):
         from xrspatial.reproject import reproject
         with pytest.raises(TypeError, match="xarray.DataArray"):
@@ -653,6 +680,20 @@ def test_merge_invalid_strategy(self):
         with pytest.raises(ValueError, match="strategy"):
             merge([raster], strategy='median')
 
+    def test_merge_rejects_irregular_input_coords(self):
+        from xrspatial.reproject import merge
+        raster = xr.DataArray(
+            np.zeros((4, 4)), dims=['y', 'x'],
+            coords={'y': [3.0, 2.0, 1.0, 0.0],
+                    'x': [0.0, 1.0, 2.3, 3.0]},
+            attrs={'crs': 'EPSG:4326'},
+        )
+        with pytest.raises(
+            ValueError,
+            match=r"merge\(\): rasters\[0\].*x coordinates.*regular.*worst step deviation",
+        ):
+            merge([raster], target_crs='EPSG:4326')
+
     def test_merge_strategy_last(self):
         """merge() with strategy='last' uses the last valid value."""
         from xrspatial.reproject import merge