How many degrees differ Grid North from True North at latitude N 30.0000 deg and longitude 35.0000 deg ?

Answer 1

The problem with grid north is that it is in alignment with true north only at the central meridian. Without knowing what type of map you are using, UTM (Universal Tranverse Mercator) or Ordnance Survey, or some other map projection like MGRS, it is impossible to answer your question. You should be able to find the grid north correction on a small diagram somewhere on your map.

Using a UTM map, the maximum correction or error is 3 degrees near the North Pole. At Lat 30, you could probably interpolate to 1 degree, but you will have to know what side of the central meridian of your UTM zone you are on to decide which way to correct. For a standard UTM zone, you are closest to the 33 degree central meridian so your correction should be about 0.66 to 1.00 degrees to the west of grid north.

Here is more explanation:
http://www.uwgb.edu/DutchS/FieldMethods/UTMSystem.htm

Here is a spreadsheet that will calculate changes on OS maps:
http://www.ordnancesurvey.co.uk/oswebsite/gps/information/coordinatesystemsinfo/gpsspreadsheet.html

Answer 2

When the curved earth is projected onto a flat piece of paper, two reference frames exist. The true north reference frame uses the latitude-longitude coordinates of the curved earth as the reference axes. In this coordinate system the longitudes (Y-axis) converge upon a single point, the Earth's rotational pole. The longitude line is the 0o azimuth direction or north direction. In the grid system (state plane or UTM coordinates) the Y axis is the direction of 0o azimuth and does not converge to a singe point. In most map projections, these two north reference directions are not the same (see the figure below), and differ by a rotation through an angle called the convergence angle.

For the explanation that works best for you, choose from these lists of information:
http://search.yahoo.com/search?p=true+north+vs+grid+north&fr=FP-tab-web-t400&toggle=1&cop=&ei=UTF-8
http://www.google.com/search?num=100&hl=en&lr=lang_en&q=true+north+vs+grid+north&btnG=Search