Lost Waterways: Clues from Digitized Historical Maps of Manila and Other Philippine Cities

We search for lost bodies of water in the cities of Manila, Tacloban, Iloilo, Cebu, Davao, and Naga by aligning their digitized Spanish-era and American-era maps to Google maps. These vanished ancient waterways can either become flooding hazards in case of extreme weather events, or liquefaction hazards, in case of earthquakes. Digitized historical maps of the cities were georectified, overlaid on current Google maps, and checked for potential missing bodies of water. Inspection through field visits and interviews with locals were conducted to verify the actual status of suspected sites. The validation identified lost, found, and even new bodies of water. There was also evidence of affected buildings, rainless flooding, and a new normal for the meaning of flooding among frequently inundated residents.


Introduction
Inland bodies of water are extremely important resources that, even if cartographers from centuries ago do not have access to aerial imagery, most likely they will depict the location of lakes and rivers with fair accuracy.
Through time some of these bodies of water would have vanished due to natural causes (e.g. sedimentation) or human activity (e.g. reclamation). Erecting structures on top of these lost bodies of water may be hazardous as these areas are prone to flooding after severe rainfall [01], or to liquefaction and amplified ground shaking during an earthquake.
In this work we aligned digitized 19 th century and 1940's maps of six Philippine cities with Google Map to determine areas where original bodies of water have vanished. We then use Google Street View to inspect if the bodies of water have indeed disappeared or were simply left out in the Google map. We visited and inspected each of the sites and interviewed locals about flooding incidents. By identifying lost waterways we can create hazard maps that can be used for disaster mitigation especially when Metro Manila and the rest of the Philippines are highly disaster-prone [03,04].
Overlaying historical maps with present maps has been done by other groups for different purposes such as historic visualization [05], urban change analysis, [07,09], and understanding environmental issues [06,08].
Our work extends the purpose to disaster mitigation by providing a map of potential flooding/liquefaction Report for Cartography of Old Informs the New (COIN) Project 2 hazards down to the street level. Like previous work we can only reliably georeference historical maps by using streets, roads, and other built structures that have remained up to the present as ground control points [02].

Digitization
Historical maps were sourced from the National Archives of the Philippines, museums, libraries, and online repositories. Hard copies of maps were digitized using the NIJI-S Scanner developed by the Advanced Imaging Science and Technology Kyoto University. The NIJI-S uses cold light (white LEDs) for illumination and a line scan camera for capture and is custom-built for digitizing fragile 2D cultural heritage objects such as old paintings and historical documents. Figure 1 shows our team performing map digitization in the School of Urban and Regional Planning Library using NIJI-S.

Georectification
We used MATLAB Mapping Toolbox to rectify the historical maps of the cities in the Philippines onto their corresponding Google maps. The toolbox uses the function cpselect to allow manual selection of common points (known as Ground Control Points or GCPs) on both the historical map and the Google map.
The function fitgeotrans creates a transformation matrix from the set of selected GCPs. The function requires an input of a transformation type. We applied projective and second order polynomial transformation types in this study. The warping and rectification of the historical map is performed by the imwarp function using the generated transformation matrices. The rectified historical map was then overlaid on Google map. The resulting overlaid map was used to compare the historical maps with the Google maps to look for missing bodies of water. The missing bodies of water were confirmed via Google Street View and fieldwork validation.

Scope of Work
Although scanning, georeferencing, and change analysis was done on several maps, we limited our fieldwork validation to six major cities in the Philippines namely: Manila City, Iloilo City, Tacloban City, Davao City, Cebu City, and Naga City. Table 1 lists the fieldwork dates per city. The objective of the fieldwork is to validate the locations of the bodies of water that disappeared according to the processed map. The teams took photographs of the specified locations and interviewed local officials and residents about flooding incidents. The validation fieldwork team discovered that an exposed culvert ( Figure 3) can be seen on top of estuary that disappeared in the 1908 and Google maps. This opens the possibility that estuaries that are absent in Google map could have been channeled underground.
In many cases during the validation fieldwork, the interviewed locals claim they do not experience flooding in their area. Yet they describe that the water level does rise during rain but lowers just as fast. If the water does not linger after the rain, and is just ankle-deep, they do not consider it flooding. It may be possible that the locals have already developed a different baseline for the meaning of flooding.
Traced in Figure 4 is a map summary of field surveyed Pasig River estuaries that are confirmed to have vanished (red), are still existing but not present in Google map (green) or may have been diverted underground (violet).   Figure 6) on areas where old tributaries of the Iloilo river were located. According to local officials the area is a high flooding risk and the civil works were part of their continuing effort to improve the drainage system. In contrast to flooding risks, community wells like the one pictured in Figure 7 were found in the area where the waterways disappeared, an indication of a freshwater source underground.  During a courtesy call with the Iloilo City mayor, the fieldwork team was also informed about a land lot along Dalan Montinola that was identified by our team to be of high risk to liquefaction. According to the mayor's personal account, a building, more than five stories high, was built on the said lot, however, only after a few months, the building was observed to be tilting. Construction was then halted until thorough inspection was made on the property. Based on the 1833 map, the present Dalan Montinola appears to be a backwater of the Iloilo River. Figure 8 shows the visited sites in Iloilo City that were confirmed to have vanished bodies of water. Figure 9 shows that there was a pond in the middle of Tacloban City in the year 1944. In the present, this pond is no longer visible. Since the area is an old body of water, locals said that the area is prone to flooding Report for Cartography of Old Informs the New (COIN) Project 8 that is why there is also an effort to make proper drainage systems as shown in Figure 10.

Davao
The missing bodies of water in the Davao maps (shown in Figure 13) are the results of changes in the meandering of the Davao River. Maps from 1944 showed the Davao River flowed towards the eastern part of the city. Presently, the river cut downwards, flowing straight to the sea. The old path of the river dried up and a community was built on top of it. Locals living in that community said that their area is prone to waist-level floods, especially during high tide. Figure 14 shows Google Street View and fieldwork photographs of the same location, in the community built on top of the dried up river. The water remnants indicate the lack of a proper drainage system so flooding has an amplified effect on the community.   Figure 16 shows three maps of Cebu City. The left map shows a large body of water extending towards the ocean on the right side of the map. In the middle figure , the waterway closed off and formed a lagoon. In Figure 9c, the lagoon disappeared completely. The fieldwork team surveyed the area of the missing lagoon and found out that there is still a body of water (shown in Figure 10) passing through the south side of the original lagoon. A whole community has taken residence in the area of the former lagoon known as Tinago.

Cebu
Interviews with locals stated that their area is lower than sealevel and has a high risk of flooding such that even if it does not rain in their area, if it rains in the mountains, their community gets flooded. A local councilor revealed that her grandmother often tells of the time when they were washing clothes in the lagoon in her younger years.  Google maps.
Report for Cartography of Old Informs the New (COIN) Project 13 Figure 18 shows the Parian Estuary in Cebu City. This image shows an example of the reduction of the size of old bodies of water, not necessarily disappearing. The vintage photograph appears to be Parian Estero which used to wide enough for bancas or small boats to pass through. The image on the right meanwhile is Parian Estero (estuary) at the time of field work. It appears narrower and shallower. Finally, Figure 19 shows the validated traces of lost bodies of water in Cebu City.   Figure 20 shows the georectified historical map of Naga City. The fieldwork team confirmed that main parts of the Naga river are still present but the islands within the river that are seen from the historical map are not.

Naga
The old waterway jutting from the river going northwards is no longer present. That waterway was believed to be man-made according to Naga City historian Dr. Danilo Gerona and was used to transport goods needed by the bishop. At the end of this waterway is the Naga Cathedral. Another observation is that the local government already started posting signs (shown in Figure 21) warning the locals about the dangers of the area. The sign directly translates to "Warning! This area is flood prone. Be cautious! Each life is precious."   Finally, shown in Figure 22 are the lost and found water ways in Naga City.
Report for Cartography of Old Informs the New (COIN) Project 16

Conclusions
By referring to digitized historical maps of Manila City, Iloilo City, Tacloban City, Davao City, Cebu City, and Naga City we were able to identify bodies of water that have shrunk in size or have disappeared altogether. We validated our findings through fieldwork surveys and interviews. We found that depiction of rivers or estuaries in Google Map is not always accurate. Possibly, this is due to the reliance on satellite imagery. Estuaries crowded around by houses or foliage get obscured from top view. As a result, we also report where waterways are actually still present. Interviews with local residents confirm that areas with lost waterways are flood prone.
It was fortunate that in more than one fieldwork the survey team observed ongoing flood control civil works which revealed that there were waterways under built roads. Thus we report the location of waterways that may have been diverted underground.
Vintage photographs of the cities were also sought from local museums and libraries. Interestingly, one should not discount hotels and restaurants because, as what we found in Hotel Alejandro in Tacloban, they can be a source of historical pictures as well.