Slide for FOSS4G Japan 2020 presentation. Copy of speakerdeck - https://speakerdeck.com/kochizufan/foss4g-japan-2020

1. Introduction of HTGCL (Historical
Topographic Ground Control Line)
- New paradigm of handling historical map in GIS -
Kohei Otsuka – Code for History
1

2. Table of contents
• What is Code for History?
• Introduction to
• Introduction of HTGCL (Historical Topographic Ground
Control Line) and its application to GIS
• Other small topic (Not related with HTGCL)
• Conclusions
2

3. What is Code for History?
3

4. A community that uses information
technology to solve historical problems
(practically one person)
https://code4history.dev/
4

5. Characteristics
• I want to emphasize that for us, information technology is a
"means" and not an “objective".
• The "objective" is to solve a historical problem, and if
necessary, to utilize non-IT means.
• The first priority is always to solve the problem.
5

6. Introduction to
6

7. Map viewer technology that allows you to
enjoy walking around the city using
historical maps and pictorial maps
© OpenStreetMap © Utouto Tanken-tai
7

8. We recently changed it’s logo 8
Maplat

9. Features of
• Not distorting historical maps
• Adjusting not only the location of the center point, but also the direction
and scale
• Bidirectional one-to-one conversion (Returning to the original display
position after multiple map changes)
• Converting lines to lines (Roads to roads, rivers to rivers, etc.)
• It's open source, so anyone can create their own site with it
• Project Sites:
Library: https://github.com/code4history/Maplat/wiki
Editor: https://github.com/code4history/MaplatEditor
9

10. Not distorting historical maps
• Existing technology (GIS) distorts historical maps
10

11. Not distorting historical maps
• overlays historical maps without distorting them
11

12. Adjusting the direction and scale
• Existing technology (Stroly) can't adjust the right orientation
and scale
12

13. Adjusting the direction and scale
• fits the direction and scale almost perfectly
13

14. Bidirectional one-to-one conversion
(homeomorphic conversion)
• Existing technology (Stroly) shifts the display position when
continuously switching maps
14

15. Bidirectional one-to-one conversion
(homeomorphic conversion)
• does not shift the display position when
continuously switching maps (Japan patent JP-6684776)
15

16. • In the past, used to get misaligned on the road at
times.
Converting lines to lines
Modern map
Historical map
16

17. • In the past, used to get misaligned on the road at
times.
Converting lines to lines
Modern map
Historical map
17

18. • Latest can accurately convert a point on the road to
a point on the road
Converting lines to lines
Modern map
Corresponding lines
(HTGCL)
Historical map
18

19. • Latest can accurately convert a point on the road to
a point on the road
Converting lines to lines
Modern map
対応線
Historical map
19
Corresponding lines
(HTGCL)

20. Introduction of HTGCL (Historical
Topographic Ground Control Line)
and its application to GIS
20

21. What is the HTGCL?
• Abbreviation for Historical Topographic Ground Control Line
• Naming of the corresponding lines between modern geographic
coordinates and historical map coordinates which are used in
's "converting lines to lines" function.
• The concept is borrowed from Ground Control Points (GCPs),
which are the points in existing GIS that correspond to
inaccurate maps and accurate maps.
• Relying on points alone for coordinate transformations between
maps makes it difficult to maintain homomorphism throughout
the transformations, and it is not possible to convert lines to lines.
Therefore, transformations using HTGCL are valuable not only
for but also for existing GIS.
21

22. Analyzing and GIS processes
• 's function can be divided into two main processes
• Converting coordinates between exact geographic coordinates and
historical map coordinates, with features such as maintaining
homomorphism and converting lines to lines => A
• Overlapping historical map and modern map according to the result
of the coordinate transformation, with adjusting the position,
direction and scale. => B
• Handling historical map with existing GIS also can be divided into
two main processes
• Converting coordinates between exact geographic coordinates and
historical map coordinates => C
• According to the results of the coordinate transformation, the
historical map is distorted, reorganized into precise geographic
coordinates and superimposed on the modern map. => D
• If 's coordinate transformation A is better than existing
GIS's coordinate transformation C, can't 's coordinate
transformation A can be exported as pre-process of D in GIS?
22

23. Comparison of HTGCL and existing GCP
in coordinate conversion
Method Rely on Pros Cons
GIS
(GDAL, Thin plate
spline)
GCP
• Relatively easy to
create data
• Smooth conversion
results
• Conversion is not
continuous or
bidirectional.
• Not converting lines
to lines
HTGCL
• Homeomorphic
conversion
• Converting lines to
lines
• Conversion becomes
sharp on the edges
of the triangle
network
• Resolving topological
errors manually is a
pain
23

24. An example of application of HTGCL
transformation to an existing GIS 24
The result used GCPs (GDAL) The result used HTGCL

25. Homeomorphic transformation 25
The result used GCPs (GDAL) The result used HTGCL
• The continuity
and bi-
directionality
of the map
coordinates is
lost, as
evidenced by
the
occurrence of
spatial loops
in some
places.
• Although
intensely
distorted, the
continuity
and bi-
directionality
of space, or in
other words,
homeomorphi
sm, is strongly
maintained.

26. Transforming lines to lines 26
The result used GCPs (GDAL) The result used HTGCL
• The green line
shows the
modern roads,
but the roads
on the
historical map
side have
been violently
distorted into
a curve and
are off the
green lines.
• The roads on
the historical
map side
have been
transformed
to follow the
green lines
definitely,
although
somewhat
distorted.

27. Cons: Sharp transformation on
the edge of triangulation 27
The result used GCP Original data used in conversion
• Transformations using
HTGCL are computed
by vector
transformations within
triangles generated by
the triangulation.
• As a result, the
transformations across
the edges can result in
sharp shape.
• We are investigating
whether there are any
measures to mitigate
this change.
Sharp conversion
The edge of triangulation which
is used for this sharp
transformation

28. Cons: The pain of manually fixing
topological errors 28
Topological errors case Fixed by adding dummy GCP
Errors
Dummy GCP
• To perform a
homeomorphic
transformation with
HTGCL, there must not
be any topological
errors in the
triangulation.
• If topological errors
exist, it can be resolved
by adding dummy GCPs
in the appropriate
places, but for now we
have to do it manually.
• We are collecting enough examples of what error cases and
where GCPs need to be added, and hope to automate
topological errors resolution in the future.

29. HTGCL based map transformation
function will be in MaplatEditor 29
• Currently, there is no solution for
converting historical maps using HTGCL to
GIS data such as WMTS.
• However, in the near future, MaplatEditor
will include the function to generate WMTS
tile data from HTGCL.
• In addition to HTGCL support, we are also
considering various functions to handle
WMTS, including support for Japanese old
GSI topographic map tiling and the
creation of new tile map that slightly shifts
the coordinate system of existing tile maps.
• In other words, MaplatEditor aims to be a GIS editor product that can be an
alternative to QGIS's Georeferencer and GDAL's gdal2tiles.py, rather than just a data
editor for .

30. Other small topic (Not related
with HTGCL)
30

31. PWA service worker framework for
caching tile map 31
• Now we are developing a Service Worker framework that
works with PWA (Progressive Web App) that can cache tile
data for tile maps offline. This framework makes it easy to
create PWA map applications that can work offline.
• It supports caching of map tiles, bulk download of caches,
and clearing of caches. The Service Worker is already
almost complete and we are developing a wrap class to
make it easy to use on the front end. At the moment it only
supports raster tiles, but if it is technically possible to
support vector tiles, we will support it.
• https://github.com/code4history/TileCacheServiceWorker

32. Conclusions
32

33. • is an open-source map viewer technology that allows you to
walk around town using historical maps.
• One of the features of is the ability to convert lines to lines in
the coordinate conversion between historical maps and modern maps.
• The corresponding lines networks those are set up when converting
lines to lines is named HTGCL.
• Since coordinate conversion using HTGCL has advantages over
coordinate conversion using GCP in existing GIS, there is also value in
creating tile map data by GIS-like historical map image transformation.
• MaplatEditor will not only serve as a data creation editor for ,
but will also enhance the ability to create GIS tile map data in the future.
Conslusions 33

34. • I have developed through my experience as a GIS engineer, and as
the antithesis to historical map processing in GIS. As a result of pursuing
that approach, I am very happy about the possibility of providing a new
paradigm on the GIS side as a result.
• I think this is also the result of developing , a solution with a
different concept than GIS, but with the base technology system of GIS in
place. I was also previously the developer of another solution called Stroly,
but Stroly was a technology that was not based on a GIS technology system,
and if I had continued to develop Stroly, I don't think we would be making
this presentation today.
• However, at any rate, is being developed by just one person, so I
have no timetable yet for when this feature will be implemented.
I have a lot of other features that I need to prioritize and develop, so it's
likely to be years in the future.
• is an open source activity, so if any GIS expert developers are
interested in this presentation, please help me.
Last comments 34

35. End
User: kochizufan
Github pages: https://code4history.dev/
E-mail: kochizufan@code4history.dev
Slide: https://speakerdeck.com/kochizufan/foss4g-japan-2020 35