Fix for very large number of samples

CHANGELOG.md

@@ -33,6 +33,14 @@ heading to indicate that only the bug fixes and security fixes are in the bug fi
 release.
 -->
 
+## [Unreleased]
+
+### Changed
+- Reduced linescan ISD ephemeris sampling from one-per-line to every 10th line for images with 1000+ lines, significantly reducing ISD file sizes and load times for large sensors. Configurable via `reduction` and `ephem_sample_rate` props. [#677](https://github.com/DOI-USGS/ale/pull/677)
+
+### Fixed
+- Fixed Eigen 5.x compatibility by removing version constraint in CMakeLists.txt [#677](https://github.com/DOI-USGS/ale/pull/677)
+
 ## [1.1.3] - 2026-03-12
 
 ### Fixed

CMakeLists.txt

@@ -31,7 +31,7 @@ else()
 endif()
 
 if(ALE_USE_EXTERNAL_EIGEN)
-  find_package(Eigen3 3.3 REQUIRED NO_MODULE)
+  find_package(Eigen3 REQUIRED NO_MODULE)
 else()
   add_library (eigen INTERFACE)
   add_library (Eigen3::Eigen ALIAS eigen)

ale/base/type_sensor.py

@@ -97,7 +97,24 @@ def ephemeris_time(self):
           ephemeris times split based on image lines
         """
         if not hasattr(self, "_ephemeris_time"):
-            self._ephemeris_time = np.linspace(self.ephemeris_start_time, self.ephemeris_stop_time, self.image_lines + 1)
+            # Determine reduction mode. Default is "linear" (subsample every
+            # Nth line). Set reduction=None via props to keep one-per-line.
+            props = self._props if hasattr(self, '_props') else {}
+            reduction = props.get('reduction', 'linear')
+            rate = props.get('ephem_sample_rate', 10)
+
+            if reduction == 'linear':
+                # Sample at most every Nth line to keep ISD file size in check
+                # for large linescan images (e.g., 177k lines for Chandrayaan-2
+                # TMC). For images under ~1000 lines, keep one sample per line
+                # to avoid reducing below 100 samples.
+                reduced = self.image_lines // rate + 1
+                num_samples = reduced if reduced >= 100 else self.image_lines + 1
+            else:
+                # No reduction: one sample per line (original behavior)
+                num_samples = self.image_lines + 1
+
+            self._ephemeris_time = np.linspace(self.ephemeris_start_time, self.ephemeris_stop_time, num_samples)
         return self._ephemeris_time
 
     @property

ale/drivers/chandrayaan_drivers.py

@@ -611,34 +611,6 @@ def sensor_frame_id(self):
 
         return self.original_naif_sensor_frame_id - 10
 
-    @property
-    def ephemeris_time(self):
-        """
-        Returns an array of times between the start/stop ephemeris times
-        based on the number of lines in the image.
-        Expects ephemeris start/stop times to be defined. These should be
-        floating point numbers containing the start and stop times of the
-        images.
-        Expects image_lines to be defined. This should be an integer containing
-        the number of lines in the image.
-
-        Returns
-        -------
-        : ndarray
-          ephemeris times split based on image lines
-        """
-
-        # Override the parent class logic, as returning the ephemeris time for
-        # every single line does not scale for Chandrayaan2 TMC images, which
-        # can have 189999 lines. Sampling at every 10th line should be enough.
-        num = self.image_lines
-        if num > 20000:
-            num = int(float(num) / 10.0)
-        if not hasattr(self, "_ephemeris_time"):
-            self._ephemeris_time = \
-              numpy.linspace(self.ephemeris_start_time, self.ephemeris_stop_time, num + 1)
-        return self._ephemeris_time
-
 class Chandrayaan2OHRCIsisLabelNaifSpiceDriver(LineScanner, IsisLabel, NaifSpice, NoDistortion, Driver):
 
     @property
@@ -814,31 +786,3 @@ def sensor_frame_id(self):
         """
 
         return self.original_naif_sensor_frame_id - 10
-
-    @property
-    def ephemeris_time(self):
-        """
-        Returns an array of times between the start/stop ephemeris times
-        based on the number of lines in the image.
-        Expects ephemeris start/stop times to be defined. These should be
-        floating point numbers containing the start and stop times of the
-        images.
-        Expects image_lines to be defined. This should be an integer containing
-        the number of lines in the image.
-
-        Returns
-        -------
-        : ndarray
-          ephemeris times split based on image lines
-        """
-
-        # Override the parent class logic, as returning the ephemeris time for
-        # every single line does not scale for Chandrayaan2 OHRC images, which
-        # can have 101075 lines. Sampling at every 10th line should be enough.
-        num = self.image_lines
-        if num > 20000:
-            num = int(float(num) / 10.0)
-        if not hasattr(self, "_ephemeris_time"):
-            self._ephemeris_time = \
-              numpy.linspace(self.ephemeris_start_time, self.ephemeris_stop_time, num + 1)
-        return self._ephemeris_time