diff --git a/src/nested/map/mod.rs b/src/nested/map/mod.rs
index 4481014..7aeb63e 100644
--- a/src/nested/map/mod.rs
+++ b/src/nested/map/mod.rs
@@ -5,6 +5,7 @@ use num_traits::{AsPrimitive, PrimInt};
 pub mod astrometry;
 pub mod fits;
 pub mod img;
+pub mod mom;
 pub mod skymap;
 
 /// `HHash` stands for HEALPix Hash.
@@ -63,7 +64,7 @@ mod tests {
         None,
         Some(ColorMapFunctionType::LinearLog), //Some(ColorMapFunctionType::LinearSqrt)
         "test/resources/skymap/skymap.png",
-        true,
+        false,
       )
       .unwrap();
   }
diff --git a/src/nested/map/mom.rs b/src/nested/map/mom.rs
new file mode 100644
index 0000000..a43bc67
--- /dev/null
+++ b/src/nested/map/mom.rs
@@ -0,0 +1,268 @@
+//! Multi-order Map, i.e. list of non-overlapping `(UNIQ, VALUE)`.
+// IDEE:
+// * use zuniq
+// * use array of tuple, sorted according to zuniq
+// * for each img pixel, get sky position, get zuniq, binay_search in map
+
+use std::{
+  cmp::Ordering,
+  fmt::{Debug, Display},
+  mem,
+  iter::Map,
+  slice::Iter,
+};
+
+use num::PrimInt;
+
+use crate::nested::map::skymap::SkyMapValue;
+
+
+/// `ZUniqHHashT` stands for `Z-curve ordered Uniq Healpix Hash Type`.
+pub trait ZUniqHashT:
+// 'static mean that Idx does not contains any reference
+'static + PrimInt + Send + Sync + Debug + Display + Clone
+{
+  /// Number of unused bits (without including the sentinel bit).
+  const N_RESERVED_BITS: u8 = 2;
+  /// Number of bits needed to code a sub-cell relative index.
+  const DIM: u8 = 2;
+  /// Number of base cells, i.e. number of cell at depth 0.
+  const N_D0_CELLS: u8 = 12;
+  /// Number of bits needed to code the base cell index.
+  const N_D0_BITS: u8 = 4;
+  /// Number of bytes available in the 'ZUniqHHashT'.
+  const N_BYTES: u8 = mem::size_of::<Self>() as u8;
+  /// Number of bits available in the 'zuniq'.
+  const N_BITS: u8 = Self::N_BYTES << 3;
+  /// Maximum depth that can be coded on this 'ZUniqHHashT'.
+  const MAX_DEPTH: u8 = (Self::N_BITS - (Self::N_RESERVED_BITS + Self::N_D0_BITS)) / Self::DIM;
+
+  fn mult_by_dim<T: PrimInt>(v: T) -> T {
+    v.unsigned_shl(1)
+  }
+  fn div_by_dim<T: PrimInt>(v: T) -> T {
+    v.unsigned_shr(1)
+  }
+  fn shift(delta_depth: u8) -> u8 {
+    Self::mult_by_dim(delta_depth)
+  }
+  fn delta_with_depth_max(depth: u8) -> u8 {
+    Self::MAX_DEPTH - depth
+  }
+  fn shift_from_depth_max(depth: u8) -> u8 {
+    Self::shift(Self::delta_with_depth_max(depth))
+  }
+  
+  /// Returns the `depth` and `hash value` this `zuniq` contains.
+  fn from_zuniq(zuniq: Self) -> (u8, Self) {
+    let n_trailing_zero = zuniq.trailing_zeros() as u8;
+    let delta_depth = Self::div_by_dim(n_trailing_zero);
+    let depth = Self::MAX_DEPTH - delta_depth;
+    let hash = zuniq >> (n_trailing_zero + 1) as usize;
+    (depth, hash)
+  }
+
+  fn depth_from_zuniq(zuniq: Self) -> u8 {
+    let n_trailing_zero = zuniq.trailing_zeros() as u8;
+    let delta_depth = Self::div_by_dim(n_trailing_zero);
+    let depth = Self::MAX_DEPTH - delta_depth;
+    depth
+  }
+
+  /// Transforms a `depth` and `hash value` tuple into a `zuniq`.
+  fn to_zuniq(depth: u8, hash: Self) -> Self {
+    let zuniq = (hash << 1) | Self::one();
+    zuniq.unsigned_shl(Self::shift_from_depth_max(depth) as u32)
+  }
+
+  fn are_overlapping(zuniq_l: Self, zuniq_r: Self) -> bool {
+    let (depth_l, hash_l) = Self::from_zuniq(zuniq_l);
+    let (depth_r, hash_r) = Self::from_zuniq(zuniq_r);
+    Self::are_overlapping_cells(depth_l, hash_l, depth_r, hash_r)
+  }
+  
+  fn are_overlapping_cells(depth_l: u8, hash_l: Self, depth_r: u8, hash_r: Self) -> bool {
+    match depth_l.cmp(&depth_r) {
+      Ordering::Equal   => hash_l == hash_r,
+      Ordering::Less    => hash_l == hash_r.unsigned_shr(((depth_r - depth_l) << 1) as u32),
+      Ordering::Greater => hash_r == hash_l.unsigned_shr(((depth_l - depth_r) << 1) as u32),
+    }
+  }
+
+}
+
+impl ZUniqHashT for u32 {}
+impl ZUniqHashT for u64 {}
+
+/// `MOM` stands for **M**ulti **O**rder healpix **M**aps.
+/// Here, it consists in a list of HEALPix cells (hash) at various depth (HEALPixordes) 
+/// with a value attached to each cell.
+/// All cells in a given MOM:
+/// * must be non-overlapping, 
+/// * have the same type of value attached to them
+/// * are sorted following the z-order curve order.
+/// This last property allow for streaming processing and operations.
+/// In practice, we use the `zuniq` index: it encodes both the depth and the cell hash value
+/// and is built in such a way that the natural order follow the z-order curve order.  
+pub trait Mom<'a> {
+  /// Type of the HEALPix zuniq hash value (mainly `u32` or `u64`).
+  type ZUniqHType: ZUniqHashT;
+  /// Type of the iterator iterating on the skymap values.
+  type ValueType: SkyMapValue + 'a;
+  /// Type of iterator iterating on all (sorted!) zuniq values.
+  type ZuniqIt: Iterator<Item = Self::ZUniqHType>;
+  /// Type of iterator iterating on all (sorted!) entries.
+  /// # Remark
+  /// We could have defined `Iterator<Item = &'a (Self::ZUniqHType, Self::ValueType)>;`
+  /// but it would have limited the possible implementations (e.g. using 2 vectors and the `zip` operation.
+  type EntriesIt: Iterator<Item = (Self::ZUniqHType, &'a Self::ValueType)>;
+
+  /// Largest depth the MOM may contain.
+  fn depth_max(&self) -> u8;
+
+  /// Returns the entry, if any, containing the given HEALPix cell hash computed at the `Mom`
+  /// maximum depth.
+  fn get_cell_containing(&'a self, zuniq_at_depth_max: Self::ZUniqHType) -> Result<Option<(Self::ZUniqHType, &'a Self::ValueType)>, String> {
+    self.check_zuniq_depth_is_depth_max(zuniq_at_depth_max)
+      .map(|_| self.get_cell_containing_unsafe(zuniq_at_depth_max))
+  }
+
+  /// Same as `get_cell_containing` without checking that `zuniq_at_depth_max` depth is the MOM
+  /// maximum depth.
+  fn get_cell_containing_unsafe(&'a self, hash_at_depth_max: Self::ZUniqHType) -> Option<(Self::ZUniqHType, &'a Self::ValueType)>;
+
+  /// Returns all entries overlapped by the HEALPix cell of given `zuniq` hash value.
+  fn get_overlapped_cells(&'a self, zuniq: Self::ZUniqHType) -> Vec<(Self::ZUniqHType, &'a Self::ValueType)>;
+
+  /// Returns all HEALPix zuniq hash, ordered following the z-order curve.
+  fn zuniqs(&'a self) -> Self::ZuniqIt;
+
+  /// Returns all entries, i.e. HEALPix zuniq hash / value tuples, ordered following the z-order curve.
+  fn entries(&'a self) -> Self::EntriesIt;
+
+  /// Check if the gieven `zuniq` depth is the MOM maximum depth.
+  fn check_zuniq_depth_is_depth_max(&self, zuniq_at_depth_max: Self::ZUniqHType) -> Result<(), String> {
+    let depth = Self::ZUniqHType::depth_from_zuniq(zuniq_at_depth_max);
+    if depth == self.depth_max() {
+      Ok(())
+    } else {
+      Err(format!("Wrong depth for zuniq {}. Expected: {}. Actual: {}", zuniq_at_depth_max, self.depth_max(), depth))
+    }
+  }
+
+  fn check_is_mom(&'a self) -> Result<(), String> {
+    let mut it = self.zuniqs();
+    if let Some(mut l) = it.next() {
+      let (mut depth_l, mut hash_l) = Self::ZUniqHType::from_zuniq(l);
+      for r in it {
+        if depth_l < self.depth_max() {
+          return Err(format!("Element has a larger depth than MOM maximum depth. Elem: {}; Depth: {}; Mom max depth: {}", l, depth_l, self.depth_max()));
+        }
+        let (depth_r, hash_r) =  Self::ZUniqHType::from_zuniq(r);
+        if l >= r {
+          return Err(format!("The MOM is not ordered: {} >= {}", l, r));
+        } else if Self::ZUniqHType::are_overlapping_cells(depth_l, hash_l, depth_r, hash_r) {
+          return Err(format!("Overlapping elements in the MOM: {} and {}. I.e. depth: {}; hash: {} and depth: {}, hash: {}.", l, r, depth_l, hash_l, depth_r, hash_r));
+        }
+        l = r;
+        depth_l = depth_r;
+        hash_l = hash_r;
+      }
+    }
+    Ok(())
+  }
+
+}
+
+/// Implementation of a MOM in a simple vector.
+pub struct MomVecImpl<Z, V>
+  where
+    Z: ZUniqHashT,
+    V: SkyMapValue
+{
+  depth: u8,
+  entries: Vec<(Z, V)>,
+}
+impl<'a, Z, V> Mom<'a> for MomVecImpl<Z, V>
+  where
+    Z: ZUniqHashT,
+    V: 'a + SkyMapValue
+{
+  type ZUniqHType = Z;
+  type ValueType = V;
+  type ZuniqIt = Map<Iter<'a, (Z, V)>, fn(&'a (Z, V)) -> Z>;
+  type EntriesIt = Map<Iter<'a, (Z, V)>, fn(&'a (Z, V)) -> (Z, &'a V)>;
+
+  fn depth_max(&self) -> u8 {
+    self.depth
+  }
+
+  fn get_cell_containing_unsafe(&'a self, hash_at_depth_max: Self::ZUniqHType) -> Option<(Self::ZUniqHType, &'a Self::ValueType)> {
+    match self.entries.binary_search_by(|&(z, _)| z.cmp(&hash_at_depth_max)) {
+      Ok(i) => {
+        let e = &self.entries[i];
+        Some((e.0, &e.1))
+      },
+      Err(i) => {
+        if i > 0 {
+          // if array len is 0, i will be 0 so we do not enter here.
+          let e = &self.entries[i - 1];
+          if Z::are_overlapping(hash_at_depth_max, e.0) {
+            return Some((e.0, &e.1));
+          }
+        }
+        if i < self.entries.len() {
+          let e = &self.entries[i];
+          if Z::are_overlapping(hash_at_depth_max, e.0) {
+            return Some((e.0, &e.1));
+          }
+        }
+        None
+      }
+    }
+  }
+
+  fn get_overlapped_cells(&'a self, zuniq: Self::ZUniqHType) -> Vec<(Self::ZUniqHType, &'a Self::ValueType)> {
+    let mut range = match self.entries.binary_search_by(|&(z, _)| z.cmp(&zuniq)) {
+      Ok(i) => i..i + 1,
+      Err(i) => i..i,
+    };
+    while range.start - 1 > 0 &&  Z::are_overlapping(zuniq, self.entries[range.start - 1].0) {
+      range.start -= 1;
+    }
+    while range.end < self.entries.len() && Z::are_overlapping(zuniq, self.entries[range.end].0)  {
+      range.end += 1;
+    }
+    range.into_iter().map(|i| {
+      let (z, v) = &self.entries[i];
+      (*z, v)
+    }).collect()
+  }
+
+  fn zuniqs(&'a self) -> Self::ZuniqIt {
+    self.entries.iter().map(|&(zuniq, _)| zuniq)
+  }
+
+  fn entries(&'a self) -> Self::EntriesIt {
+    self.entries.iter().map(|(z, v)| (*z, v))
+  }
+}
+
+
+/*
+
+pub struct Mom {
+  pub depth_max: u8,
+  pub elems: MomElems,
+}
+
+pub enum FitsMomElems {
+  U64U8(Vec<(u64, u8)>),
+  U64I16(Vec<(u64, i16)>),
+  U64I32(Vec<(u64, i32)>),
+  U64I64(Vec<(u64, i64)>),
+  U64F32(Vec<(u64, f32)>),
+  U64F64(Vec<(u64, f64)>),
+}
+
+*/
diff --git a/src/nested/map/skymap.rs b/src/nested/map/skymap.rs
index df0f590..700573a 100644
--- a/src/nested/map/skymap.rs
+++ b/src/nested/map/skymap.rs
@@ -9,12 +9,11 @@ use std::{
   slice::Iter,
 };
 
-use crate::n_hash;
 use colorous::Gradient;
 use mapproj::CanonicalProjection;
 use num_traits::ToBytes;
 
-use crate::nested::map::HHash;
+use crate::{n_hash, nested::map::HHash};
 
 #[cfg(not(target_arch = "wasm32"))]
 use super::img::show_with_default_app;
@@ -97,14 +96,23 @@ impl SkyMapValue for f64 {
 }
 
 pub trait SkyMap<'a> {
+  /// Type of the HEALPix hash value (mainly `u32` or `u64`).
   type HashType: HHash;
+  /// Type of the value associated to each HEALPix cell.
   type ValueType: 'a + SkyMapValue;
+  /// Type of the iterator iterating on the skymap values.
   type ValuesIt: Iterator<Item = &'a Self::ValueType>;
+  /// Type of the iterator iterating on the skymap entries.
+  /// WARNING: we are so far stucked with iterator on ranges,
+  /// e.g `(0..n_cell).iter().zip(...)`, since it relies on the `Step` trait
+  /// which requires `nightly builds`.
+  /// In the case of `implicit` skymaps, a solution is to use `enumerate`.
   type EntriesIt: Iterator<Item = (Self::HashType, &'a Self::ValueType)>;
 
+  /// Depth (<=> HEALPix order) of the skymap.
   fn depth(&self) -> u8;
 
-  /// Tells wether the map is implicit or not.
+  /// Tells whether the map is implicit or not.
   /// If implicit, method `values` and `entries` will return as many items as the number
   /// of HEALPix cell at the map HEALPix depth.
   fn is_implicit(&self) -> bool;