Grid Cell Origin

What is a 2D Grid? A 2D grid is a regular spatial discretization of an area into equally sized cells.
Each cell represents a spatial unit for which values (e.g. elevation, thickness, parameters) are defined. A crucial aspect of a grid definition is how the position of each cell is referenced.

---

Grid Cells vs. Grid Points A common source of confusion is the distinction between:

• Grid cells (areas)

• Grid points (coordinates)

In GeoScene3D, grid-based data are internally defined using grid points, not full polygons.

---

Grid Origin in GeoScene3D GeoScene3D defines the grid origin (null point) at the lower-left corner of the grid. This means:

• The grid coordinates start in the lower-left corner

• Cell spacing is applied in positive x- and y-directions from this point

This definition is consistent with many numerical and interpolation-based grid formats.

---

Cell Reference Point: Center of the Cell Although the grid origin is located at the lower-left corner, GeoScene3D defines grid values at the center of each cell. In practice, this means:

• The first grid value is located half a cell size away from the lower-left corner

• Each grid point represents the center of a grid cell, not its corner

This is the key point that often causes confusion.

---

Effect on Visualization Because grid values are defined at cell centers, the visualization behaves as follows:

• The first cell is centered around its grid point

• At the grid boundary, only half of the first cell lies inside the grid extent

As a result:

• The first grid cell appears visually as a quarter cell in the lower-left corner

This is not an error, but a direct consequence of:

1. the grid origin being defined at the corner, and
2. the grid values being defined at cell centers.

---

Illustration The figure below illustrates this behavior:

• Grid points are located at the centers of the cells

• The grid origin is at the lower-left corner

The first visible cell is therefore only partially shown This visual effect helps explain why the grid may appear “cut” at the edges.