INTEGRATING GIS ACROSS THE UNITED STATES-MEXICO BORDER: THE TIJUANA RIVER WATERSHED PROJECT

In early 1994, the National Oceanic and Atmospheric Administration 

provided startup funding to San Diego State University (SDSU) and 
 
El Colegio de la Frontera Norte (COLEF) for the Tijuana River 

Watershed (TRW) project to address concerns related to this

 watershed which is shared by Mexico and the United States. 

This led to a formal agreement in late 1994 that laid out the 

guidelines for the coordination between SDSU and COLEF in 

the development of a comprehensive GIS for the TRW.


The agreement provides for data sharing, coordinated data
 
development and scientific research, and joint use of the 

data for education and research. The project has five complementary 

components: GIS database development, community outreach, 

education (university and K-12), scientific, and watershed 

management. In this paper the authors describe the geographical 

characteristics of the watershed, major issues in the watershed, 

the project goals, and the technical issues involved in creating 

and implementing a binational transborder GIS. 



INTRODUCTION   

With startup funding from the National Oceanic and Atmospheric 

Administration (NOAA) and subsequent support from the United 

States Geologic Survey (USGS) and the United States Department 

of Defense (USDOD), San Diego State University (SDSU) and 

El Colegio de la Frontera Norte (COLEF)/L'Institut Francais 

de Recherche Scientifique pour le D(veloppement en Coop(ration 

(ORSTOM) are jointly preparing a GIS for the Tijuana River 

Watershed (TRW). The purpose of the GIS is to address a 

set of environmental concerns related to the watershed 

which is shared by Mexico and the United States.



Cooperation between SDSU and COLEF has been formalized 

via an agreement that provides for data sharing, coordinated 

data development and scientific research, and joint use of 

the data for education and research. This paper will provide 

an overview of the characteristics and major issues in the 

watershed, the project components, and the issues involved 

in integrating geospatial data across the United States-Mexico 

border. Four examples of data integration-hypsography, 

hydrography, geology, and soils- illustrate the challenges 

(and possible solutions) of creating a transborder GIS.



PROJECT SETTING   

The Tijuana River Watershed, covering an area of 1,725 

square miles, two-thirds of which is in Mexico, lies astride 

the California-Baja California section of the United States-Mexico 

border (see Figure 1). The watershed is a diverse geographical 

area with a wide range of topography, climates, biological 

resources, land uses and social-political institutions. It is 

also located within the San Diego - Tijuana region which 

has a population of over four million that is expected to 

increase to nearly five million by the end of the century. 

Abutting the border on the U.S. side is the Tijuana River 

estuary, the largest remaining functioning wetland in 

southern California which provides critical habitat for 

birds on the Pacific flyway.



The watershed and contiguous area are a quagmire of differing 

agendas, cultures, economic classes and political systems. 

Mexico is struggling to improve its economy with tremendous 

growth occurring in the region due to increasing development 

in the economic zone along the border. The San Diego - Tijuana 

region expects to receive the greatest impact from NAFTA- 

related growth. This will occur in an urban system which 

lacks the basic infrastructure to support such growth. Although 

the two countries share many watersheds, the international 

boundary has reinforced an approach in which both countries 

address social and environmental problems as national 

responsibilities, rather than international obligations. 

The United States can afford to place environmental protection 

as a high priority on its political agenda. Mexico, however, is 

overwhelmed by the current and projected demands on its 

limited infrastructure and resources.



ENVIRONMENTAL ISSUES

In November 1994, a binational workshop was held to identify 

important planning and environmental issues in the watershed. 

Many of the issues were concerned with environmental pollution 

and data and communication insufficiencies.



Pollution of the surface and subsurface waters is a significant 

problem. Sewer lines and treatment systems exist in the 

watershed, but they are not adequate to process the large 

volumes of industrial and domestic wastes that are 

generated. The effects of pollution are most noticeable 

in the Tijuana River estuary and adjoining coastal waters 

which are the unwilling recipients of pollution generated 

upstream. The international sewage treatment under 

construction on the U.S. side, which is intended to treat 

Tijuana sewage, will reduce the frequency of sewage flows 

in the river resulting from breakdowns in the Mexican systems. 

However, it does not address many causes of pollution in the 

basin such as those caused by industrial dumping, poor 

agricultural practices, and flows from residences not 

connected to the sewage system. The domestic sewer 

system continues to be expanded, but not at a pace fast 

enough to keep up with the rapid expansion of population 

and urban and industrial land uses. Rapid urbanization is 

also resulting in a depletion of natural habitat and a greater 

need for imported water and energy resources, and increasing 

degradation of air quality. 


Increasing the intractability of pollution problems, related 

to population growth and inadequate pollution measures, 

is the lack of a coordinated geospatial data infrastructure 

for the watershed. In addition to the scarcity of key data, 

there are numerous transborder data inconsistencies. Many 

local, state, and federal agencies on both sides of the border 

have responsibilities in the basin. With few exceptions, 

however, there is little binational coordination among 

these agencies as they carry out their mandated responsibilities. 

From time to time, binational meetings are held to discuss 

common problems in the region, but generally there is little 

meaningful follow-up. In general, substantial asymmetries 

in technology, culture, and funding continue to exist. Poor 

communications make it difficult to overcome these 

differences.



PROJECT COMPONENTS

The project has five complementary components: technological

(GIS database development), community outreach, bilingual 

education and awareness, scientific studies, and watershed management.



GIS Data Development

In the first phase of the project, emphasis is on the creation 

of basic layers in the database. Other data will be added 

later in response to the requirements of specific applications. 

Thematic elements include hypsography, hydrography, geology, 

geomorphology, vegetation, soils, land use, climate, and census 

boundaries, most at scales and resolutions equal to map scales 

of 1:24,000 to 1:250,000. The main purpose of the GIS is to 

develop a spatially referenced socio-environmental database 

to facilitate education, scientific studies, and decision making 

within the concept of shared management of the watershed. 

This part of the project involves significant binational 

collaboration, with researchers from SDSU and COLEF/ORSTOM 

meeting on a weekly basis.



Community Outreach

Efforts are underway to develop a strong community 

outreach component so that the GIS is not produced in a 

social-political vacuum. In addition to holding user needs 

assessment workshops, the primary partners have enlisted 

more than 20 other entities from federal, state, and 

local levels from both sides of the border. Also, the 

project team is hoping to attract funding to hire a 

coordinator who would be responsible for interacting 

with members of the public, government, and private sectors.


Education

Bilingual education for a wide range of students and the 

lay public is an essential component of the project. 

Subject to the availability of funding, educational 

initiatives planned for the project include the following: 

a video of the project and the watershed, a digital flyover 

through the watershed, one or more interactive multi-media 

educational modules using ArcView, a large poster of the 

watershed, a customized package of the database and 

ArcView on a CD, and color and monochromatic atlases 

of the watershed.



Scientific Studies

Numerous research initiatives have been identified that 

will receive attention in the near term. They include 

environmental and toxics risk assessment, water and 

air pollution analysis, multiple species habitat modeling, 

evaluation of service and infrastructure needs, and land use analysis.



Watershed Management

Effective resource management policies require a binational, 

participatory decision-making process that incorporates 

the temporal changes expected to occur throughout the 

watershed as a result of economic development. The GIS 

will be used to facilitate a dialogue among watershed 

stakeholders about the possible future impacts of 

human activities and resource distribution in the 

watershed. In conjunction with this component, there 

will be an investigation of the feasibility of establishing 

a binational non-profit organization. The primary purpose 

of this entity would be to sponsor activities leading to 

improved management practices in the watershed.



DATA INTEGRATION ISSUES

The issues involved in integrating already existing geospatial 

data across the U.S.-Mexico border are both numerous and complex.

In this section we briefly describe the general categories of 

data incompatibility which we have had to address on this project.



Scale and Generalization 

First and foremost are the difficulties of generating a uniform 

database from maps and other sources that, in some instances, 

differ widely in scale and generalization. Although most of the 

source data fall within the 1:24,000-1:50,000 range, which is 

suitable for many of the proposed applications, some data are 

at a scale of 1:250,000  Data at the latter scale are highly 

generalized, and must be used cautiously when combined with 

larger scaled, more detailed spatial objects.


Data Classification

Several layers in the database contain categorical information. 

Harmonizing data of this type requires that one select from a 

number of alternatives. For example, one could employ (a) the 

classification employed on U.S. maps, (b) the classification 

employed on Mexican maps, (c) some combination of the 

previous alternatives, and (d) a "neutral" classification 

scheme that permits crosswalking from one country's 

classification to the other. All four alternatives have 

been employed on this project.



Census Subdivision Size 

Social-economic data are collected for census subdivision 

of various sizes in the two countries. For the project database, 

roughly comparable enumeration units were generated by 

digitizing U.S. Census tracts and Mexican BasicGeostatistic Areas (AGEBs).



Datums and Ellipsoids 

Source maps employed for the database are based on two datums 

- NAD 27 and NAD 83. In some parts of the watershed the 

horizontal shift between NAD 27 and NAD 83 amounts to 

about 30-40 meters. This difference is resolved by converting 

NAD 27 coordinates to the NAD 83 datum. In addition to different 

datums, the use of several ellipsoids were encountered 

during the course of the project - the Clarke 1866, Clarke 

1880, and the GRS 1980. Horizontal differences of about 

50 - 60 meters were found to exist between the Clarke 

1866 and Clarke 1880 ellipsoids.Eventually all data 

were converted to the GRS 80 ellipsoid.


Temporal Variation 

Data sources for the GIS vary in age from early 1950s to 1995. 

Whenever feasible, older mapped data were updated to 1995 

using NOAA 1:50,000 color photography and SPOT satellite imagery.


Positional Accuracy

The positional accuracy of the different layers in the database 

varies depending in part on the nature of the data. Some of the 

features, e.g. vegetation polygons have fuzzy boundaries. Map 

scale is also a determinant with positional accuracy of digitized 

data decreasing with decreasing map scale. To the degree 

possible, we have tried to maintain a positional accuracy 

of about 20 - 50 meters. This is achieved through the use 

of terrain corrected and georeferenced SPOT panchromatic 

imagery, with a pixel size of 10 meters, as the base to 

which other layers are referenced.



Data conversion between GIS Software Packages

Two different GIS packages are employed for the project. 

On the Mexican side, researchers at COLEF use the ORSTOM 

software called SAVANE. On the U.S. side, SDSU uses ArcInfo. 

Conversion of data  between the two packages has been 

achieved with minimum difficulty, and is usually accomplished 

through the use of Generate Files. Occasional difficulties in 

data conversion have arisen. For example, data from SAVANE 

retained portions of bounding polygons associated with the 

edges of map sheets used as data sources. These came across 

as straight line artifacts that divided some of the polygons. 

These artifacts were cleaned interactively. 	 



EXAMPLES OF DATA INTEGRATION

Hypsography

To create the hypsography layer, a high quality digital 

elevation model (DEM) was created from large scale 

topographic maps. The resulting DEM allows the 

generation of a wide variety of products such as slope 

steepness, slope aspect, contours, shaded relief, 

and sub-basins.



United States Section

Digital data were derived from scanned USGS 1:24,000-scale 

contour mylar separates. Of the 18 quadrangles that comprise 

the U.S. portion of the watershed, 16 have a 40-foot contour 

interval and two have a 20-foot interval. ArcInfo software 

was used to convert the scanned contours to GRID format 

and then to vectorized contour lines. The contours were 

edited to correct scanning problems. Afterwards, elevation 

values and other information such as depression contours, 

carrying contours, and supplementary contours were added. 

This process yields a high quality digitized contour map that

 meets USGS quality standards.


Mexican Section

The elevation database was generated by manually digitizing 

contour lines on the 13 INEGI 1:50,000-scale paper map sheets 

that comprise the Mexico section. These maps have printing dates

 ranging from 1974 to 1980 and a contour interval of either 20 

meters (eleven sheets) or 10 meters (two sheets). Some rubber 

sheeting was necessary to edgematch the sheets.


Harmonization

The first step in this phase was to generalize the contour 

lines to reduce the number of points which would be used 

for interpolation. This was necessary because of the large 

number of points in the contour maps and limitations 

within ArcInfo. ArcInfo's GENERALIZE command was 

used to generalize the lines. After line generalization, 

the elevation values were converted from feet to meters 

so that x, y, and z coordinates would be in the same type 

of units. The TOPOGRID command in ArcInfo was used to 

generate 15-meter DEM's for each quadrangle. The separate 

DEM's were merged using the GRID Mosaic command which 

results in smooth transitions between quadrangles by 

computing the mean value of grid cells that overlap 

between the quadrangles.



Hydrography

The hydrography layer has been created from existing USGS 

and INEGI topographic maps.



United States Section

All blue line features - perennial and intermittent- were 

digitized from the USGS 1:24,000-scale quadrangles that 

comprise the U.S. portion of the watershed. These maps 

carry compilation dates ranging from 1955 to 1972.



Mexican Section

The perennial and intermittent blue line features were 

digitized from the INEGI 1:50,000-scale maps that comprise 

the Mexican portion. The maps were compiled during the 

period 1974 to 1980.  Editing was done using NOAA 

1:50,000-scale color aerial photography dated 1994 

and a 1995 SPOT satellite scene which is a merge of 

10-meter panchromatic and 20-meter multispectral imagery.



Harmonization

The INEGI maps contain a much higher density of intermittent 

stream symbols than their U.S. counterparts. Clearly, different 

decision criteria were employed in symbolizing hydrography, 

given that the two sides of the border do not vary significantly 

in their climatic and gelogic characteristics. Additionally, 

many blue line symbols that should connect at the border, 

do not when the INEGI and adjoining USGS maps are juxtaposed. 

Two steps were followed to symbolize a transborder hydrographic 

network that is uniform in density and integrated across the border. 

	

1) First, blue line features on the INEGI maps were classed 

according to stream order (1 to 7) with 1 representing source 

tributaries and 7 representing main streams. After a period 

of trial and error experimentation, streams of order 1 with 

a length of less than 925 meters were eliminated from the 

coverage. This resulted in a blue line density that was more 

or less uniform across the border.

	
2) Where appropriate, the blue line features were connected 

at the border to correctly represent transborder drainage patterns. 


Geology

Existing geology maps for the watershed vary greatly in 

terms of their scale and geologic classifications.  Some 

1:24,000-scale and 1:50,000-scale coverages are available 

for the U.S. and Mexican portions, respectively. However, 

1:250,000 is the largest scale for which there is complete 

coverage of the watershed. Even at this scale, however, 

the available maps do not provide a uniform format and 

geologic classification. The geologic maps chosen as 

sources for the database were those which (1) exhibited 

roughly uniform levels of generalization,(2) demonstrated 

the greatest degree of transborder matching of polygons, 

and (3) covered contiguous areas, on the chance that the 

database may be extended beyond the TRW in the future.


Harmonization

To harmonize the U.S. and Mexican map sources it was 

necessary to first unify the different geologic classifications 

and then conduct polygon matching along the international 

border. The types of harmonization performed included 

correcting situations in which 1) similar rock types 

were classed differently, 2) different rock types were 

classed as the same type, and 3) polygons of the same 

type did not match along the border. While some limited 

fieldwork and aerial photograph interpretation were 

conducted to generate the combined database, additional 

field checking and revision of the catagories would allow 

a further refinement of the database.



Soils	

The soils maps of the U.S. and Mexican portions of the 

watershed are incompatible in terms of generalization 

and the type of soils classifications employed. Consequently, 

considerable effort has been expended in determining the 

best approach for integrating diverse types of soils data 

to produce a unified soils layer.



United States Section

Soils polygons were digitized from the U.S. Soil Conservation 

Survey 1:24,000-scale maps. The classification employed 

on these maps is the Seventh Approximation, a hierarchical 

system in which soils are classed according to soil properties.

 Six levels-order, suborder, great group, subgroup, family, 

and series- comprise the classification system.


Mexico Section

The soils polygons in the Mexican section were digitized 

from 1:250,000-scale INEGI maps on which the soils are 

grouped according to the Food and Agricultural Organization. 

This system is based on a two-fold classification which is 

much less detailed than that employed on the Soil Conservation 

Survey maps.



Harmonization

The Mexican and U.S. soils maps are harmonized in two 

different ways. The first product is a highly generalized 

representation created by using the great group categories 

from the soils maps on the U.S. side, which is approximately 

similar to the primary categories of the FAO classification 

used on the Mexican maps. The second, still under preparation, 

is a logit model that equates soil types on the U.S. side with 

certain physical parameters, e.g. climate, vegetation, topography, 

and geology. This model will be applied to the Mexican portion, 

resulting in a soils database corresponding to a scale of 1:50,000.



CONCLUSIONS

In summary, the Tijuana River Watershed GIS project illustrates 

the difficulties of harmonizing geospatial data across the United 

States-Mexico border. The solutions obtained for developing this 

database are applicable to other transborder GIS projects.


REFERENCES

Wright, R., Ries, K. and Winckell, A. Identifying Priorities For a GIS For the Tijuana River Watershed: Applications for Land Use Planning and Education, Workshop Proceedings, Institute For Regional Studies of the Californias, San Diego State University, San Diego, CA, 1955, 93pp.

Askov, D., Wright, R., and Greenwood, N. "Predictive Mapping of the Soils of the Tijuana River Watershed", GIS/LIS '95, November 1995, Nashville, TN.

Wright, R. and Wright, P. "The Tijuana River Watershed GIS Project: A Binational Consortium," Proceedings, GIS/LIS '95, November 1995, pp.1046-1055.



AUTHOR INFORMATION

Richard D. Wright

Professor of Geography

Department of Geography

San Diego State University

San Diego, CA 92182-4493

Ph.: (619) 594-5466

FAX: (619) 594-4938

E-mail: wright@typhoon.sdsu.edu