diff --git a/ifes_apt_tc_data_modeling/ato/ato_reader.py b/ifes_apt_tc_data_modeling/ato/ato_reader.py
new file mode 100644
index 0000000..6e635d2
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+++ b/ifes_apt_tc_data_modeling/ato/ato_reader.py
@@ -0,0 +1,117 @@
+# POS file format reader used by atom probe microscopists.
+#
+# Copyright The NOMAD Authors.
+#
+# This file is part of NOMAD. See https://nomad-lab.eu for further info.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+#
+
+# pylint: disable=no-member,duplicate-code
+
+import os
+
+import numpy as np
+
+from ifes_apt_tc_data_modeling.nexus.nx_field import NxField
+
+from ifes_apt_tc_data_modeling.utils.mmapped_io import get_memory_mapped_data
+
+
+class ReadAtoFileFormat():
+    """Read Rouen group *.ato file format."""
+
+    def __init__(self, filename: str):
+        assert len(filename) > 4, "ATO file incorrect filename ending!"
+        assert filename.lower().endswith(".ato"), \
+            "ATO file incorrect file type!"
+        self.filename = filename
+
+        self.filesize = os.path.getsize(self.filename)
+        self.number_of_events = None
+        self.version = None
+        retval = self.get_ato_version()
+        if retval in [3, 4, 5]:
+            self.version = retval
+            print(f"ATO file is in a supported version {self.version}")
+            if self.version == 3:
+                assert (self.filesize - 2 * 4) % 14 * 4 == 0, \
+                    "ATO v3 filesize not integer multiple of 14*4B!"
+                self.number_of_events = np.uint32((self.filesize - 2 * 4) / (14 * 4))
+                print(f"ATO file contains {self.number_of_events} entries")
+            if self.version == 5:
+                assert (self.filesize - 5000) % 40 == 0, \
+                    "ATO v5 filesize not integer multiple of 40B!"
+                self.number_of_events = np.uint32((self.filesize - 5000) / 40)
+                print(f"ATO file contains {self.number_of_events} entries")                
+        else:
+            raise ValueError("ATO file unsupported version!")
+        # https://zenodo.org/records/8382828
+        # details three versions of the Rouen/GPM ato format v3, v4, v5
+        # Cameca/AMETEK's runrootl/FileConvert utility know two ATO flavours:
+        # CamecaRoot v18.46.533g built Marc, 21, 2022 against ROOT 5.34/36
+        # v3 LAWATOP and v5 current GPM
+
+    def get_ato_version(self):
+        header = get_memory_mapped_data(self.filename, "<u4", 0, 4, 2)
+        if header[1] in [3, 4, 5]:
+            return header[1]
+        return None
+
+    def get_reconstructed_positions(self):
+        """Read xyz columns."""
+
+        xyz = NxField()
+        xyz.typed_value = np.zeros(
+            [self.number_of_events, 3], np.float32)
+        xyz.unit = "nm"
+
+        if self.version == 3:
+            xyz.typed_value[:, 0] = \
+                get_memory_mapped_data(self.filename, "<f4",
+                                    2 * 4 + 0 * 4, 14 * 4, self.number_of_events)  # wpx -> x
+            xyz.typed_value[:, 1] = \
+                get_memory_mapped_data(self.filename, "<f4",
+                                    2 * 4 + 1 * 4, 14 * 4, self.number_of_events)  # wpy -> y
+            xyz.typed_value[:, 2] = \
+                get_memory_mapped_data(self.filename, "<f4",
+                                    2 * 4 + 2 * 4, 14 * 4, self.number_of_events)  # fpz -> z
+        if self.version == 5:
+            xyz.typed_value[:, 0] = \
+                np.float32(get_memory_mapped_data(self.filename, "<i2",
+                                    5000 + 0, 40, self.number_of_events))  # wpx -> x
+            xyz.typed_value[:, 1] = \
+                np.float32(get_memory_mapped_data(self.filename, "<i2",
+                                    5000 + 2, 40, self.number_of_events))  # wpy -> y
+            xyz.typed_value[:, 2] = \
+                get_memory_mapped_data(self.filename, "<f4",
+                                    5000 + 4, 40, self.number_of_events)  # fpz -> z
+        return xyz
+
+    def get_mass_to_charge_state_ratio(self):
+        """Read mass-to-charge-state-ratio column."""
+
+        m_n = NxField()
+        m_n.typed_value = np.zeros(
+            [self.number_of_events, 1], np.float32)
+        m_n.unit = "Da"
+
+        if self.version == 3:
+            m_n.typed_value[:, 0] = \
+                get_memory_mapped_data(self.filename, "<f4",
+                                    2 * 4 + 3 * 4, 14 * 4, self.number_of_events)
+        if self.version == 5:
+            m_n.typed_value[:, 0] = \
+                get_memory_mapped_data(self.filename, "<f4",
+                                    5000 + 8, 40, self.number_of_events)
+        return m_n