SME
Geographic Information Systems (GIS)
Vector and raster data
Use this for “A 30 km × 30 km scene in 30 m cells, 2 bytes a cell: how many columns, rows and cells, and how big is the file?”
Diagram
Columns
=
1,000
×
30
km
30
m
=
1,000;
rows
=
1,000
×
30
km
30
m
=
1,000.
Cells
=
1,000
×
1,000
=
1,000,000;
1,000,000
×
2
bytes
10
6
=
2
MB.
Each square drawn is 50 × 50 cells.
Halving c to 15 m would make 4,000,000 cells, four times as many.
This is an example. Type your own values to start a new problem.
W
Extent width
example · km
H
Extent height
example · km
c
Cell size
example · m
nc
Columns
worked out (not typed)
nᵣ
Rows
worked out (not typed)
N
Cells
worked out (not typed)
B
Bytes per cell
example
S
File size
worked out (not typed) · MB
Formulas
nc
=
1,000
×
W
c
1,000
=
1,000
×
30
30
nᵣ
=
1,000
×
H
c
1,000
=
1,000
×
30
30
N
=
nc
×
nᵣ
1,000,000 = 1,000 × 1,000
S
=
N
×
B
1,000,000
2
=
1,000,000
×
2
1,000,000
Assumptions
•
No compression and one band: every cell stores one value of B bytes; a megabyte is 10⁶ bytes.
•
Square cells c on a side, and the extent is a whole number of cells across and down.
•
Halving the cell size makes four times the cells, and a file four times as big.
Step-by-step
WE KNOW
W = 30 km, H = 30 km, c = 30 m, B = 2
FIND
columns (nc), rows (nᵣ), cells (N), file size (S)
Step 1 · Find columns (nc)
Use nc = 1,000 × W ÷ c
The
width
in
metres,
cut
into
cells
c
wide.
nc
=
1,000
×
30
30
nc
=
30,000
30
nc = 1,000
Step 2 · Find rows (nᵣ)
Use nᵣ = 1,000 × H ÷ c
The
height
in
metres,
cut
into
cells
c
tall.
nᵣ
=
1,000
×
30
30
nᵣ
=
30,000
30
nᵣ = 1,000
Step 3 · Find cells (N)
Use N = nc × nᵣ
A cell for every column in every row.
N
=
1,000
×
1,000
N
=
1,000,000
Step 4 · Find file size (S)
Use S = N × B ÷ 1,000,000
Each
cell
stores
B
bytes;
a
megabyte
is
a
million
bytes.
S
=
1,000,000
×
2
1,000,000
S
=
2,000,000
1,000,000
S
=
2
MB
Check
Put the numbers back in. Both sides should match.
1,000
=
1,000
×
30
30
✓
1,000
=
1,000
×
30
30
✓
1,000,000 = 1,000 × 1,000 ✓
2
=
1,000,000
×
2
1,000,000
✓
Topic check
See what you know: 5 questions on this topic
New questions each time. Pass to mark it complete.