Israeli Transverse Mercator

The Transverse Mercator projection is generally applied to regions that are long in the north–south dimension, and for large areas two or more zones may be established to limit distortion.[3] Israel applies ITM in a single national grid. ITM uses the GRS80 reference ellipsoid, a scale factor of 1.0000067, and a fictitious central meridian; these unusual parameters were selected to minimize changes from the older grid rather than to follow conventional projection parameters.[4][1]

The Israel State Archives retention schedule lists "Israel Grid and New Israel Grid" as a Survey of Israel record category designated for permanent preservation.[5] ITM coordinates also appear on civilian products such as the 2012 Israel National Trail map series — a four-map set at 1:40,000 scale — where New Israel Grid coordinates are printed in red on the cover.[6]

Before ITM, Israeli mapping relied on a grid built around a Cassini–Soldner projection and a triangulation network whose origins lay in a survey campaign begun in 1921 under the British Mandate Survey Department.[4] That classical network served Israel's cartographic needs for several decades, but improvements in electronic distance measurement exposed the older grid's accuracy limitations, and it eventually ceased to meet operational requirements.[4]

A readjustment of classical geodetic observations — including Laplace azimuths, horizontal angles, and distances — produced the foundation of the New Israel Grid. That readjustment extended through fourth-order networks and encompassed approximately 24,000 triangulation points.[1] Coordinates for the three highest network orders were released in 1993 and were used mainly for government cadastral projects.[1] Surveying regulations published in June 1998 gave the New Israel Grid legal status, and from July 1998 every new geodetic project in Israel was required to comply with it.[1]

A central design goal behind ITM was to minimize disruption to existing mapped coordinates. The scale factor of 1.0000067 and the placement of a fictitious central meridian were selected together so that old and new grid coordinates would differ by less than one metre across northern Israel — the region containing approximately 95 percent of the country's population — meaning coordinate grids already printed on maps would not need to be redrawn.[1]

The grid's origin is a fictitious point designated 882M, placed approximately 700 metres west of the former grid's origin point 82M.[4] To distinguish new coordinates from old and reduce confusion between the axes, the system adds 50 km to the eastward coordinate and 500 km to the northward coordinate, while retaining six-digit metre values for both.[4] The central meridian was shifted away from an existing triangulation point near Jerusalem to a nonexistent point with an arbitrary longitude, providing the small rotation needed for the required coordinate alignment.[1]

The readjusted classical network offered significantly improved but still bounded accuracy. The highest-order points adjusted together achieved approximately 5 cm at one standard deviation, while fourth-order triangulation points reached approximately 10 cm.[4] In practice, separate local GPS projects tied to different New Israel Grid control points could differ from one another by 10–20 cm — an inconsistency that fell short of the cadastral objective of locating land boundaries to 5 cm at a 95 percent confidence level.[1]

Retaining the existing ITM projection and mapping equations was nonetheless judged preferable to introducing another wholesale coordinate-system change, which would have required transforming cadastral boundary coordinates stored in land-registry databases.[1]

To overcome the accuracy ceiling of the classical network, the Survey of Israel established the Israeli Active GPS Network (GIAN), comprising 13 permanent GPS stations.[1] Direct connection to GIAN, or the use of Virtual Reference Station data together with datum transformations, was intended to deliver ITM coordinates for new control points at centimetre-level accuracy. The official seven-parameter transformation was designed to make the resulting coordinates independent of which permanent station a project used.[1]

The 2005 paper distinguishes the projection from the improved grid and its datum: ITM is the projection, Israel 2005 (IL05) is the improved plane-coordinate system based on adjusted GPS-station coordinates, and Israel Geodetic Datum 2005 (ILGD05) is the fixed coordinate set for those stations in ITRF2000.[1] ILGD05 was referenced to GPS day 275 of 2004, or October 1, 2004, and the plane-coordinate framework was derived through a seven-parameter similarity transformation.[1]

The 2017 paper uses the source-specific designations Israel Grid 2005 (IG05) for the plane-coordinate system and Israel Geodetic Datum 2005 (IGD05) for its underlying datum. It defines IG05 on the GRS80 ellipsoid using ITM mapping equations, while IGD05 fixed the coordinates of the Active Permanent Network in ITRF2000 at reference epoch 2004.75.[2] The paper also reports a 2012 datum update, designated IGD05/12, which excluded four stations showing significant movement relative to the network and based the updated datum on 17 stations on the Sinai subplate.[2]

Israel lies near the meeting area of the African, Arabian, and Eurasian tectonic plates, and geodetic-station positions consequently change over time. Relative motion along the Dead Sea Fault is approximately 3–6 mm per year.[2] Fixed coordinates referenced to 2004 therefore ceased to represent current station positions, and the 2012 datum update did not, in the 2017 authors' assessment, fully address the continuing movement of newly measured points and boundary marks.[2]

To address this, the authors proposed a time-dependent transformation from ITRF05 to the fixed IG05 plane-coordinate system. The proposed transformation uses 14 parameters — the seven conventional similarity-transformation parameters plus their time derivatives — together with a national velocity field, allowing measured coordinates to be related to the fixed plane-coordinate system while accounting for ongoing plate motion.[2]