Israeli Cassini-Soldner Coordinate System

The Israeli Cassini-Soldner coordinate system was the projection framework of the old Israeli Grid, which dated to 1920. It used Cassini-Soldner spheroidal coordinates with a central meridian passing through a major triangulation point near Jerusalem. Regulations issued in June 1998 replaced the old grid with the New Israeli Grid and its Israel Transverse Mercator projection.[1]

The old Israeli Grid dates to 1920.[1] During the following decade, the Department of Surveys built an extensive fixed-point and detailed-mapping infrastructure across Palestine.[2]

By 1930, a decade of theodolite work had produced a fixed-point network of remarkable scale. A British report on Palestine published that year recorded 20,873 points with fully computed coordinates, alongside a further 22,898 control points and landmarks fixed by graphic means.[2] The network was organized into principal, third-order, topocadastral, fourth-order, and graphic survey tiers, reflecting the layered precision requirements of different mapping tasks.[2]

Field operations in 1930 alone extended the network substantially. Topocadastral triangulation was carried out in the hill areas, with work conducted between Lake Tiberias and Metullah, between Jerusalem and Hebron, and between Nazareth and Tiberias. During that year, 51 third-order points covering 650 square kilometers and 485 topocadastral points covering 165 square kilometers were established, along with 73 kilometers of traverse lines containing 593 points.[2]

Detailed mapping at the 1:10,000 scale accompanied the triangulation effort. Sheets were produced for parts of the coastal plain, the Carmel range, the Jezreel Valley, and areas near Tulkarem, Bittir, and Bir-el-Zait. By that time, the total area surveyed at that scale reached 5,522,000 dunums, encompassing the maritime plain, the plains of Acre and Esdraelon, and nearly the entire Jordan Valley.[2]

The old Israeli coordinate system used Cassini-Soldner spheroidal coordinates. Its central meridian passed through an existing major triangulation point near Jerusalem, tying the grid to a surveyed control point.[1]

While the system served cadastral and topographic needs for decades, the transition to a new grid in the late 1990s revealed certain inherited characteristics that shaped how the replacement was designed. The coordinate-system change involved a shift of 50 kilometres on the y axis and 500 kilometres on the x axis between the old and new grids, with meter-level differences that were irregular—amounting to several decimetres in northern Israel and up to several metres in the south.[1]

New survey regulations for Israel were officially issued in June 1998, with a central provision replacing the old Israeli Grid with the New Israeli Grid (NIG).[1] The NIG resulted from a readjustment of classical geodetic observations, including Laplace azimuths, horizontal angles, and distances, and introduced the Israel Transverse Mercator (ITM) projection in place of the Cassini-Soldner system.[1]

The ITM projection was designed with continuity in mind. Its parameters were chosen to keep coordinate differences between the old and new grids below one metre across northern Israel—a region containing approximately 95 percent of the population—with the aim of preserving existing grid lines on large-scale maps and remaining usable where approximately one-metre accuracy was sufficient.[1] The new system employed a fictitious central meridian with an arbitrary longitude rather than retaining the old meridian through a physical triangulation point, facilitating the required rotation.[1]

A 2005 Survey of Israel technical paper gives the ITM scale factor as 1.0000067 and describes the value as unusual.[1] The Survey of Israel chose to retain the existing projection and mapping equations rather than revising the parameter as part of another substantial coordinate change. The rationale was practical: a large correction could require transforming all cadastral boundary coordinates and risk creating serious errors if users failed to identify correctly which coordinate system a given point belonged to.[1]

The NIG readjustment was carried through the fourth-order network and encompassed approximately 24,000 triangulation points. The accuracy of a fourth-order point was estimated at better than 10 centimetres standard deviation relative to the nearest first-order point.[1] Coordinates for the top three orders were released in 1993 and were used primarily for governmental cadastral projects; from July 1998 onward, every new geodetic project in Israel was required to comply with the NIG.[1]

The Survey of Israel's GPS-based surveying methodology was built on the NIG framework that superseded the Cassini-Soldner system. Field procedures required at least three control points with NIG coordinates to define a local datum, with free local GPS networks adjusted to those control points using four- or seven-parameter transformations and ITM projection equations.[1] The source states that direct connection to the permanent GPS network, or use of a Virtual Reference Station combined with datum transformations, should yield ITM coordinates for new control points with centimetre-level accuracy.[1]