Nowadays, a GNSS application is one of the main backbones of life style. The GNSS signal that come from space satellites to receivers on Earth meets some error sources. The troposphere layer is one of the basic sources that cause a delay to the signal. To detect the delay, many models have been produced taking into consideration the location of the receiver and the zenith angle of the satellite. Most of these models give vertically perfect models without looking for the horizontal asymmetry in troposphere delay. The delay that happens to a signal at the north direction may be a little different than that at south direction. The horizontally graded difference "azimuth asymmetry" is negligible at high elevation angles up to 10˚. At low elevation angles, the asymmetry is larger and significant. At Z=88˚ (E=2˚), the asymmetry is about 72 mm for east and west and about 66 mm for south. Using the Precise Tropospheric Delay Database (PTD), a study over the difference in delays in the horizontal plane is carried out. PTD has the ability to calculate the delay at any azimuth angle. A new model is proposed and compared to PTD calculations. The model has three coefficients (a, b and c) obtained for high zenith angles (70˚, 80˚, 84˚, 86˚ and 88˚). The three coefficients are modeled as a function of zenith angle. These coefficients are used to give a horizontal gradient (HG) factor which is multiplied by the northern mapping function to map it to any azimuth. The results show that the delay at the eastern and western directions is greater than the delay at the southern direction which is greater than the delay at the north. The new proposed model makes significant improvement. For east and west directions, the model decreases the bias from around 72 mm to be about 1.0 mm while Herring model gives a 55 mm error. For the southern direction, the produced model needs an improvement to get closer to the true asymmetry. It reduces the bias from 66 mm to 55 mm where Herring model gives an error of 52 mm approximately.