Zoom Levels and EndLevel¶

The Garmin IMG format organizes map data into zoom levels. Each level corresponds to a coordinate resolution and determines which features are visible when the user zooms in or out on their GPS. Configuring levels correctly is essential to produce a performant and legible map.

Key concepts¶

Resolution (bits)¶

The resolution of a level is expressed in bits (1 to 24). The higher the value, the more detailed the level:

Bits	Precision/unit	Approx. GPS range	Typical usage
24	≈ 2.4 m	25 m – 350 m	Trails, buildings, maximum detail
23	≈ 4.8 m	50 m – 700 m	Neighbourhood, village
22	≈ 9.5 m	100 m – 1.5 km	Town, municipality
21	≈ 19 m	200 m – 3 km	Inter-municipal
20	≈ 38 m	400 m – 6 km	Small department
18	≈ 152 m	2 km – 23 km	District, department
16	≈ 610 m	6 km – 90 km	Region, large territory
14	≈ 2.4 km	25 km – 350 km	National view

Formula

1 map unit at N bits = 360 / 2^N degrees. At the latitude of France (~46°), 1 degree latitude ≈ 111 km. The "approx. GPS range" corresponds to the range of display scales where the Garmin firmware uses the N-bit level (10× to 150× the size of one map unit).

Levels¶

Levels are numbered starting from 0 (the most detailed). Each level is associated with a resolution:

--levels "24,20,16"

Creates 3 levels:

Level	Resolution	GPS zoom
0	24 bits	Most zoomed in (maximum detail)
1	20 bits	Intermediate zoom
2	16 bits	Most zoomed out (wide view)

EndLevel (in the .mp file)¶

Each feature (road, building, contour...) carries an EndLevel attribute that defines up to which level it remains visible:

[POLYLINE]
Type=0x01
EndLevel=2
Data0=(45.18,5.16),(45.19,5.17)
[END]

The rule is simple: a feature with EndLevel=N is visible at levels 0 to N.

EndLevel / Levels correspondence¶

With `--levels "24,20,16"` (3 levels)¶

EndLevel	Visible at levels	Resolutions	Copies in the file
0	0 only	24	×1
1	0, 1	24, 20	×2
2	0, 1, 2	24, 20, 16	×3

With `--levels "24,22,20,18,16"` (5 levels)¶

EndLevel	Visible at levels	Copies
0	0	×1
1	0, 1	×2
2	0, 1, 2	×3
3	0, 1, 2, 3	×4
4	0, 1, 2, 3, 4	×5

Impact on file size

Each additional copy increases the IMG file size. A feature with EndLevel=7 in a 9-level configuration is written 8 times. This is the primary lever for controlling output size.

Recommendations¶

Number of levels¶

Levels	Usage	Size impact
2 (`"24,18"`)	Simple map, minimum size	Reference
3 (`"24,20,16"`)	Good size/navigation compromise	+30-50%
5 (`"24,22,20,18,16"`)	Detailed navigation	+100-150%
9 (`"24,23,...,16"`)	Theoretical maximum	+200-400%

Recommendation for BD TOPO

3 to 4 levels with significant resolution jumps (4-6 bit gaps) offer the best compromise for a department. For a quadrant (FRANCE-SE, SO, NE, NO), the production configuration uses 7 levels 24/23/22/21/20/18/16: levels 23 and 21 densify the detailed zone for smooth panning without bloating wide zooms.

Production 7-level configuration (France quadrants)¶

The france-quadrant/ production configuration uses the header 24/23/22/21/20/18/16. The table below shows the correspondence between index n, resolution, GPS range and visible categories:

n	bits	Approx. GPS range	Production EndLevel	Visible categories
0	24	25 m – 350 m	≥ 0 (all)	Everything: buildings, trails, roads, contours…
1	23	50 m – 700 m	≥ 1	Excluding features with EndLevel=0 (small local roads, buildings)
2	22	100 m – 1.5 km	≥ 2	Same, excluding EndLevel≤1
3	21	200 m – 3 km	≥ 3	Primary roads, rail, hydrography, vegetation
4	20	400 m – 6 km	≥ 4	Main roads (motorways → departmental), rail
5	18	2 km – 23 km	= 6 only	Municipalities, residential areas, vegetation, toponymy
6	16	6 km – 90 km	= 6 only	Municipalities, residential areas, vegetation, toponymy

Max road EndLevel = 4

In the quadrant configuration, roads (motorways, national, departmental) have EndLevel: "4". They disappear at zooms n=5 and n=6 (6 km+). Only structural polygons (municipalities, forests, urban areas) and toponymy remain visible at wide zoom (EndLevel: "6").

EndLevel by feature category¶

The table below provides optimized EndLevel values for a 3-level configuration (--levels "24,20,16"):

Category	Garmin type	EndLevel	Justification
Motorways	0x01	2	Visible at all zooms
National, departmental roads	0x04, 0x05	2	Structural network
Municipal roads	0x06, 0x07	1	Visible at medium zoom
Tracks, trails	0x0a, 0x16	0	Detail only
Main watercourses	0x1f	2	Landmarks at all zooms
Streams	0x18	0	Detail only
Large water bodies	0x3c, 0x29	2	Visible everywhere
Small water bodies	0x40-0x44	0	Detail only
Buildings	0x13	0	Detail only (all scopes)
Forests	0x50	1	Visible at medium zoom
Master contours (25m)	0x22	1	Visible at medium zoom
Intermediate contours (10m)	0x21	0	Detail only

Consistency between MP header Levels and `--levels`¶

The .mp files generated by mpforge contain a header with the zoom levels:

[IMG ID]
Levels=2
Level0=24
Level1=18
[END]

The --levels option of imgforge replaces these values. It is recommended to maintain consistency:

If the header declares Levels=2 with Level0=24, Level1=18, use --levels "24,18" or --levels "24,20,16" with adapted EndLevels
EndLevels in features should never exceed the number of levels - 1. An EndLevel=7 with only 3 levels has no more effect than EndLevel=2
If you change the number of levels, readjust the EndLevels in the transformation rules

Complete example¶

3-level configuration optimized for BD TOPO¶

mpforge rules (in garmin-rules.yaml):

# Motorways: visible at all zooms
- match:
    CL_ADMIN: "Autoroute"
  set:
    Type: "0x01"
    EndLevel: "2"    # levels 0, 1, 2

# Trails: detail only
- match:
    NATURE: "Sentier"
  set:
    Type: "0x16"
    EndLevel: "0"    # level 0 only

# Master contours: medium zoom
- match:
    IMPORTANCE: "1"
  set:
    Type: "0x22"
    EndLevel: "1"    # levels 0, 1

imgforge compilation:

imgforge build tiles/ \
    --levels "24,20,16" \
    --output gmapsupp.img \
    --jobs 8

Multi-Data: coupling level ↔ bucket¶

A feature can carry multiple geometries (Data0= very detailed, Data2= simplified for medium zoom, etc.). imgforge selects the appropriate bucket at render time. The n index of a LevelSpec in generalize-profiles.yaml corresponds directly to the index in MpHeader.levels:

Index `n`	Header	Bucket emitted	Consumed by imgforge at
`0`	`Level0=24`	`Data0=`	very detailed zoom (`Level0`)
`2`	`Level2=20`	`Data2=`	medium zoom (`Level2`)
`4`	`Level4=16`	`Data4=`	coarse zoom (`Level4`)

Fail-fast constraint: max(n) across all profiles must be < header.levels.len() — otherwise imgforge silently drops out-of-range buckets. mpforge validates at load_config and fails with an explicit message.

See mpforge — multi-level profiles and Step 2 — Multi-level profiles.

Estimating size impact¶

For a mountainous department (Isère, 169 tiles):

Configuration	Estimated size	Compilation time
9 levels, max EndLevel 7	~460 MB	~35s
3 levels, max EndLevel 2	~150-180 MB	~15-20s
2 levels, max EndLevel 1	~120-150 MB	~10-15s

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