Zoom Explained

This article explains the 2^Nth zoom scale mechanism employed in the DSRC MAPs for both the XY and LL coordinate systems.  The reader is again directed to clause 11 of the J2735 standard where zoom is discussed.

Background

The SAE DSRC MAP system uses a scaling system wherein all points align at either 1/10th of one micro degree (in the LLH coordinate system) or at 1 centimeter (in the XYZ coordinate system) .  This is the  1:1 ratio or the “non-zoom” level supporting the finest resolution. The max value (zoom=15) causes a shifting of 1:32768.  And as described elsewhere, these two coordinate systems are joined at key anchor points.

By convention, there is no zoom in intersection maps  (the zoom value is zero, hence the map is a 1:1 scale). In the absence of the a zoom value, the J2735 standard states that the scale is taken to be 1:1.

The zoom used in a given environment (use case) can be increased by orders of magnitude, reflected in a scale number.  The resolution of a given offset is then cut in half, while the range (or span) of the offset is doubled.

See clause 7.231 DE_Zoom in the current J2735 standard for full details.  See also clause 11.7.3 On the Use of Zoom and Scales for additional key commentary .

Quick Details

As a worked example, consider applying zoom to the LL offset data concept DF_Node_LL_32B and its elements which is show below

Node-LL-32B ::= SEQUENCE {
    -- ranges of +- 0.0032767 degrees
    -- ranges of +- 362.31873 meters at the equator
    lon  OffsetLL-B16,
    lat  OffsetLL-B16
    }
OffsetLL-B16 ::= INTEGER (-32768..32767)
    -- A range of +- 0.0032767 degrees
    -- In LSB units of 0.1 microdegrees (unless a zoom is employed)

In the non-zoom case this data concepts allows a (LL) span of about 362.32 meters with an precision of 1.1106 centimeters. To repeat the key item: adding zoom increases range at the expense of precision.

If the zoom is set =2 then scaling of 2^2 = 4 occurs and this allows an increased span of about 1449.26 meters with a reduced precision of 4.423 centimeters.

If the zoom is set =15 (its largest allowed value) then scaling of 2^15 = 32768 occurs. The increased span is about 107.37 degrees (~60% of the way around the Earth at the equator)  or 11,8725 km with a reduced precision of 362.33 meters.

A worked Example

Presume for some reason you needed to use the above data concept to describe a location or point that was 30km away.  You would proceed to select a scale of at least 7 (2^7=128 and a scale of 82 is needed here).  As a result, the precision with which you could describe the point would also decrease to 1.42 meters.

As a general rule, small zoom values will suffice for most use cases.

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