Basic Tiling - Achieving the Desired Maplet Overlap

(compiled by DL)

Purpose: Basic tiling allows the user to generate a suite of maplets at a higher resolution which tile a region of interest. The overlap of the new maplets is controlled by the ratio of maplet ground sample distance (GSD) to bigmap scale. This 'How-To' explains how the user achieves the desired maplet overlap percentage.

Generating a Tiling Bigmap

The user must generate a bigmap which covers the region of interest and whose parameters are set up such that the new maplets will overlap by the desired amount.

Bigmap .in File

The user must adjust the bigmap pixel scale in the tiling .in file in order to achieve the desired maplet overlap ratio. This is done by applying a multiplicative factor to the maplet ground sample distance (GSD) as follows:

where F is the multiplicative overlap factor, such that:

For example:

To achieve a 25% overlap:

maplet GSD = 10cm
bigmap scale = 10cm * 1.5 = 15cm

Example set of multiplicative factors:

Multiplicative Factor	Maplet Overlap	Example for Maplet GSD = 10cm
0.5	75%	bigmap scale = 10cm * 0.5 = 5cm
0.8	60%	bigmap scale = 10cm * 0.8 = 8cm
1.0	50%	bigmap scale = 10cm * 1.0 = 10cm
1.3	35%	bigmap scale = 10cm * 1.3 = 13cm
1.5	25%	bigmap scale = 10cm * 1.5 = 15cm
2.0	0	bigmap scale = 10cm * 2.0 = 20cm

Example BGMAP1.in (for maplets with a GSD of 10cm and a 33% overlap) :

m <- To locate, reference to a mapSTART1 <- Reference map name 49 49 <- Reference map pixel/line location 0.00013 130 1.23400 1000 <- scale (km/pixel), Q size, integer random seed, maximum maplet scaleBGMAP1 <- bigmap name1 <- bigmap program commands.005.02510011111100

A Closer Look at GSD Ratio and Overlap

GSD Ratio - Unity

When the bigmap and maplet GSDs are equal, the maplets overlap by 50% and the outer edges of the outer maplets coincide with the edge of the tiling area.

GSD Ratio - Small Overlap

When the bigmap GSD is greater than the maplet GSD, the maplets overlap by <50% and the outer edges of the outer maplets lie within the edge of the tiling area.

GSD Ratio - Large Overlap

When the bigmap GSD is smaller than the maplet GSD, the maplets overlap by >50% and the outer edges of the outer maplets lie outside of the edge of the tiling area.

The 'Dead Zone'

When the bigmap GSD is greater than the maplet GSD (F>1), there is a gap between the outer edges of the maplets and the tiling area, here refered to as the ‘dead zone’.

The width of this zone can be calculated as follows:

where, S = tiling bigmap ground sample distance; F = the multiplicative overlap factor, such that:

Unless the user is careful, he may fall into a problem across tilings where the outer edge of the maplets do not align, and, as the ground sample distance of the maplets decreases, the outer edge moves further from the tiling area center, encroaching on an area which is not included in the higher GSD maplets, as illustrated below.

If it is necessary to maintain alignment of the outermost maplet edges across tilings, adjust the tiling half q-size as follows:

where,

q = tiling bigmap half q-size;
S = tiling bigmap ground sample distance;
F = the multiplicative overlap factor, as above.
Subscripts 1, 2 indicate separate tilings which it is assumed will have different tiling bigmap ground sample distances, and an equal overlap factor, F.

The resulting tiling bigmap width can be found from: w=2qS. Note that the tiling bigmap width will change across tilings.