GIS_Merging_Land_Sea_Elevation.pdf

8/14/2019 GIS_Merging_Land_Sea_Elevation.pdf

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Principles for Bringing Land and Sea Data Together

Mark Mackenzie and Andrew Hoggarth

CARIS

Fredericton, [email protected]@caris.com

Abstract

This paper will focus on issues relating to the merging of land and marine geospatial data. Muchof the focus of spatial data infrastructure (SDI) creation and management has been on

topographic, or land, databut there is an emerging focus on the inclusion of marine data tocomplete the picture at the national, regional and global level. The objective to combine land

and marine data is made more difficult because of the different data standards applied in these

two areas.

Disparities between scale, symbology and datum cause various data integration issues when

these datasets are joined. Interoperability issues related to reconciling these differences are

heightened where shore-based and sea-based datasets meet in a coastal zone.

By combining topographic and hydrographic datasets, land and marine data integration issues

can be addressed more easily. Bringing this data together can occur either through the Webfrom disparate sources or through a harmonized interoperable central database. A benefit of

the central database is the ability to apply a one feature one time approach which allows the

integrity of each specific dataset to be maintained with the added efficiency of storing objectsonly once. By storing multiple representations along with the individual objects, we can ensurethat the data appears as expected and can therefore be used appropriately.

Successfully addressing the issues associated with merging land and sea data results in moreefficient implementation of initiatives such as coastal flood visualization, disaster management

and response, and/or Integrated Coastal Zone Management (ICZM). It is envisaged that themanagement of other geospatial data like that used for aviation purposes could also benefit

from this approach.


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Introduction

SDI building can be a difficult and intimidating task for the uninitiated, with both technological

and organizational challenges. However, seamlessly merging land and sea data together in an

SDI for projects like integrated coastal zone management adds to the challenge. This paper willconcentrate on these technological challenges and suggest methodologies to merge both land

elevation data and seafloor bathymetry data into a continuous elevation model or topobathysurface on which analysis and good decision-making can be performed.

In addition to creating topobathy surfaces, geospatial technologies can also be used to mergeland and sea map features to create new data products, such as specialized coastal zone

maps. Using modern data conflation and management techniques, features can have differentdisplay characteristics associated to them thus allowing the same feature to be used in multiple

thematic products. This paper will examine how CARIS production database technology can be

used to achieve interoperability, with land and marine features being used to create traditionaland new hybrid product offerings from the same source.

The technological challenges of merging land and sea datasets can largely be addressed by

todays geospatial software, but several decisions need to be made as to how the technologywill be applied. Decisions about vertical datum rectification and resolution differences need to beagreed upon before the technology can accurately merge these varying datasets. Additionally,

some cartographic decisions on which features to include from each dataset need to be madelargely based on the end use of the dataset.

Theoretical Considerations

Before diving into the technological solutions to these challenges, the initial focus will be on the

theoretical answers. These answers will become the basis of how todays geospatial technology

is applied to the problem. Internet research reveals a lot of information on the theory of merginggeospatial datasets on different datums, particularly in the area of rectifying the geoid and

ellipsoid, as well as dealing with resolution differences. There is little guidance however relatedto determining which cartographic features to include in products required for the coastal zone.

Reconciling differing Vertical Datums

Vertical reference surfaces can be categorized under three general headings; tidal, geodal andellipsoidal. Traditionally, bathymetric data has been collected and stored relative to a tidal

datum and topographic data relative to a geodetic datum. The introduction of precise Global

Navigation Satellite Systems (GNSS) such as GPS, GLONASS and (eventually) GALILEO, hasmade it possible to collect both land and sea data relative to a mathematically derived ellipse.

Nautical charts are produced for safety of navigation. Bathymetric data displayed on those

charts are referenced to a vertical datum where the water surface will not normally go below.This Chart Datum is usually the Lowest Astronomical Tide (LAT) or Mean Lower Low Water

(MLLW). Topographic data, on the other hand, are often referenced to a local geodetic datum,

approximated by Mean Sea Level (MSL), which is above LAT and MLLW. A geodetic datum isa continuous surface that varies with gravity. A chart datum is referenced to a low waterdetermination relative to a localized area, and differs from chart to chart.

In order to merge bathymetric and topographic data sets, it is necessary to consider these

vertical datum discrepancies. In some cases, data sets can be adjusted by simply applying aconstant offset. In other cases it is necessary to apply more complex algorithms taking into


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account sea surface topography and hydrodynamic ocean models. If the difference between

references is relatively small, within the vertical uncertainty of the datasets themselves, then itcan be ignored.

In the future, as more land and sea datasets are collected using precise GNSS the merging ofdatasets will be much less cumbersome. However, it will still be necessary to transform this

information to physical surfaces, such as the geoid, for analysis and map/chart production.

Two organizations that have developed processes for transforming between the various verticaldatums are the National Oceanic and Atmospheric Administration (NOAA) in the United Statesand the United Kingdom Hydrographic Office (UKHO).

NOAA has developed the VDatum1tool set to transform datasets between standard vertical

datums. VDatum covers the transformation of various vertical datums in three groups: tidal,

orthometric (relative to geoid) and ellipsoidal datums. This tool is limited to areas and datumswhich have a vertical transform model available and is largely limited to high traffic areas off thecoast of the continental United States. VDatum has evolved as part of NOAAs initiative to

create a framework of standards and tools in support of organizations that wish to create

topobathy surfaces.

The UKHO has been developing a vertical datum transformation framework called the Vertical

Offshore Reference Model (VORF). This framework aims to model the relationship betweenChart Datum, which is largely based on tidal levels at Lowest Astronomical Tide (LAT) and other

vertical reference surfaces, such as topographic DEM. VORF incorporates various validation

references, including satellite altimetry, geoidal models, tide gauge data throughout the UnitedKingdom, and GPS derived ellipsoidal heights and bathymetric models.2

One limitation of these vertical datum models is their current limited coverage. While each is

useful for their target areas, they are regional in nature. There is no global vertical datummodel or transformation standard that is accurate for use at the regional or local level. A global

vertical datum model would aid in the building of a master surface, with very large sets ofbathymetry surfaces for vast areas. With current enterprise database solutions and computing

power, a global bathymetry surface is a reasonable expectation from a technical standpoint, butthe rectification of the vertical reference will be more problematic.

While rectifying between ellipse and geoid references is relatively simple with todays modelsand technologies, the rectification between chart datum and geoid is more of a challenge.

Initially, chart datum must be first transferred to MSL and then MSL transferred to the geoid,

which requires extensive oceanographic modeling and examination of local tide gauge data. Itis the complexity of the process that has limited VDatum coverage to regional, largely high

traffic areas. All of these factors depend on the accuracy the user is looking for, as Geoid andMSL can vary by up to 1.5 m.

Vertical datum models can be applied to land and marine DEMs in specialized software like the

CARIS!BDB.

Horizontal Datum ModelsWhen creating a topobathy surface, if the topographic and bathymetric datasets have been

referenced to different horizontal datums then it is easy to reconcile these differences throughalready well-defined transformation mechanisms.


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Most geospatial software products include a re-projection capability, allowing users to specify

the desired horizontal reference for the data and transform the dataset on the fly. This allowsusers to import their topographic and hydrographic datasets into their desired toolset and then

pick a common horizontal reference for the two datasets. One of the more common horizontal

reference systems is the World Geodetic System, 1984 (WGS84), which is used as thereference coordinate system for GPS data collection.

Resolution Differences

The resolution and data collection method for land and sea datasets differs between disciplines.Land elevation data typically originates from the digitization of elevations from stereo aerialphotographs or from airborne terrestrial LiDAR measurements. Hydrographic data is often

derived from sonar measurements or bathymetric LiDAR. Modern sonar systems are capable ofcollecting very high-resolution data; higher than is usually digitized or collected by LiDAR. The

differences in resolution present a challenge when combining datasets from these various

sources.

Ideally, the best resolution data available from the land and the sea areas would be fused

together, with decimation to coarser resolution levels being appropriate when performing more

regional analysis.

While it is less of an issue with topobathy surfaces, resolution differences are an important

consideration when dealing with vector map or chart features. Combining vector datasets whichwere compiled at various scales requires careful consideration, as features are represented

differently with varying degrees of generalization. If vector features are derived from a topobathy

surface that has been derived from high resolution elevation and bathymetry data, then theresolution of the resultant features would also be high.

Feature Determination

The fusion of land and sea datasets also presents other cartographic challenges. Whilstcontours, sounding and spot heights can be derived from the seamless DEM, other features

need to be selected for inclusion from existing maps or charts. This merger of features requiresa decision about which features best represent certain aspects of the littoral zone.

For example, topographic maps provide feature rich detail of the land areas but a low level of

information about the sea areas. Navigational charts provide a great deal of information aboutthe sea areas but have fairly limited detail of the land areas. Both products would contain acoastline but often this would have been defined by different means and potentially represent a

different water level. Features, like landmarks, exist on both maps and chartstherefore when

building a specialized combined product from existing source data, a decision needs to be madeabout which landmarks to keep and also how they should be cartographically portrayed to

provide the appropriate information.

If a data centric geospatial database, similar to CARIS HPD, is used to store the data, thenfeatures on the land or the sea can be used for multiple purposes. For example a coastal zone

stakeholder may not be interested in safety of navigation issues for shipping, but they can utilize

nautical chart information to create enhanced products that relate to their direct need. Thepresentation of a shoaling rock on a chart, could also be used to represent the location of animportant seabird colony on an environmental product. The rock would still exist as the source

feature, but its representation could change depending on the product.

Coastline Delineation


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When using map or chart data as a source for the coastline on a specialized product, a decision

needs to be made on whether to use the coastline of the map (biased high) or the coastline ofthe chart (biased low). If a seamless DEM has been created, then effectively the coastline can

be defined as the contour boundary between positive and negative elevations. Although using

this zero value as a coastline delineator can be dangerous as often the intertidal area is poorlydefined.

With any coastline delineation, the end use of the coastline must be identified, as different

usages determine the approach required. For example, a legal boundary is often based on thehigh water mark, while a chart coastline should be based on low water, as it could be used forsafe navigation.

Whether using existing map or chart data, or a topobathy surface to delineate the coastline, it is

important to carry accurate metadata with the feature or dataset. This should include details

about the coordinate reference information and should facilitate future vertical adjustments orextractions.

Technological Methods

The challenge of bringing land and sea datasets together has resulted in the creation ofspecialized GIS software that allows users to merge these datasets in an intelligent, harmonized

and accurate way. Enterprise geospatial technologies, which utilize object-orientatedarchitecture, make it possible to combine all manner of thematic sources in a single source

database or data warehouse.

Computing power, innovative methods for handling very large and dense datasets (such as theCARIS CSAR Framework3), and modern sensor capabilities coupled with more efficient data

collection methods have resulted in an increased use of high-resolution datasets as a source for

decision-making and product creation. Terabytes of elevation measurements can now bemanaged and manipulated easily and efficiently. These factors make the production of high-

resolution topobathy surfaces possible.

Resolving Vertical Datum DifferencesAs discussed earlier, the resolution of differing vertical datums between land and sea datasets

is best accomplished by the application of vertical datum models, such as those used inVDatum or VORF. References needed for the creation of a vertical datum model includereference height data from tidal gauges, GPS reference heights and similar elevation

references.

It may also make sense to store seamless DEMs in reference to the ellipsoid as opposed to a

tidal datum. This would facilitate any subsequent vertical transformations that may be requiredwhen creating specialized products. This also ties in with the modern practice of hydrographic

surveying in reference to the ellipsoid, which has been made possible due to the availability ofprecise GNSS.4The downside of this approach is that the elevation values are heights or

depths above the ellipsoid, which does not provide immediate context when browsing visually

the DEMs.

Merging DEMs Positive and Negative Together

Bathymetric measurements on a nautical chart are usually collected as positive elevations, andsubsequently, soundings that have negative values are considered to be drying heights or

points above the chart datum. Spot heights on a topographic map are also considered aspositive elevations and points below sea level are considered to be negative. If software is to


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be used to combine land and sea datasets together then decisions need to made about what

constitutes a positive and/or negative elevation and datasets may need to be manipulated onimport to resolve signage conflicts. Software also needs to allow bathymetry data and

topographic data to co-exist rather than assuming it all to be bathymetry or all elevations above

sea level. This has been a drawback of many of the mapping systems on the market.

Case Study: Merging Land and Sea DEMs in Musquash Harbour, New BrunswickTo better illustrate some of the issues and approaches presented in this paper, the following

section will highlight the steps taken to create a topobathy DEM for a coastal region.

The area of interest was Musquash Harbour, located on the Bay of Fundy in the province of

New Brunswick, Canada. The area has some of the largest tidal fluctuations on earth. The goalof the project was to create a seamless seafloor to mountain top DEM for the area, with a view

to deriving a continuous set of vector contours.

The data used was acquired from two separate sources.

The bathymetry data of Musquash Bay and the entrance of Musquash River were obtained from

the Ocean Mapping Group of the University of New Brunswick as part of their Project on theSaint John Harbour approaches.5 The bathymetry obtained was an ASCII 'Long Lat Depth' fileat a10-metre resolution.

The land data was orthometric spot height data with an average distance between heights of

around 70 metres. The data came from the New Brunswick provincial government, through the

Service New Brunswick agency. 6

The software used to complete this project was CARIS BDB.

In order to get a sense of our area of interest, several background, or framework, datasets areused to set the coverage area. Orthophotos and topographic line files were used to visualize the

Musquash Bay and surrounding area. The lines representing the coastline are identified andseparated from the other files and used to create a boundary file to be used as a coverage area

and breakline in the TIN surface creation process later in the process.

Step one was to import the raw bathymetry and spot height data and create TIN surfaces fromeach. CARIS BDB provides an import wizard, which allows users to import any of a variety ofelevation data types, including XYZ text files, ASCII files, or CSAR Framework point clouds.


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Fig 1.1: 70m Land elevation data and 10m bathymetry data imported

Once the two raw point datasets were imported, it was clear that the datasets were referencedto different vertical datums, and would have to be rectified to a common vertical datum.

The bathymetry used is referenced to Chart Datum (LAT) and the spot height data is referencedto the local geodetic datum (~MSL). The average tidal change for this area of the Bay of Fundy

is 8 meters. Thus the bathymetric dataset was shifted -8 metres, or 8 metres up, using thesurface shift tool in CARIS BDB. This shift is somewhat arbitrary, and will introduce some errorinto the topobathy surface, but for this proof of concept was deemed acceptable. This shift

covers the entire chart datum to geodetic datum transformation process.

Once the vertical datum shift has been addressed, surfaces can be derived from the raw data

and merged into a seamless coverage. The first step was to create TIN models for eachdataset, which will become the basis for the DEM surfaces. In order to set the boundaries of the

individual surfaces, the breakline and coverage area files derived from the topographic line datawill be used.

The coverage area cuts the land TIN and is used to control the limits of the TIN model based onphysical boundaries such as coastlines, islands, or other physical structures. A breakline isused to insert known elevations/depths into the TIN. Breakline objects can be either line or area

objects. For example, a breakline could be digitized along a portion of a coastline, with the

depth attribute set to zero metres, located where it is feasible to have the model extend fully to


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the shoreline. Once applied to the TIN, the model would include the vertices of the breakline,

and the value of their depth attribute, extending the coverage of the TIN to the shoreline. This isan important step in building a complete shore-to-shore model when the bathymetry data does

not extend all the way to the shoreline.

Fig 2 Land TIN model before coverage areaapplied

Fig 2.1 Land TIN Model after coveragefeature applied

Once the TIN models are derived and the breakline and coverage areas are applied, DEMsurfaces can be built for each dataset. At this point the resolution difference between the landdataset (70m) and the bathymetry data (10m) can be addressed. CARIS BDB allows users to

set the desired resolution of their surface, so the 70m data was used to interpolate a 10m

surface on land, to match the bathymetry resolution. While this makes for a less certain TINmodel, it does allow for aesthetically better contours to be derived


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Fig 3Separate land and sea DEMs interpolated to 10m resolution.

At this point the two surfaces have been created at the same resolution, and referenced to a

common vertical datum; so they are ready to be combined using the combine surfaces functionwithin CARIS BDB. In order to combine the surfaces, they need to be deconflicted to determinewhich surface is given priority in overlapping areas.


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Fig 4 Surface combined to create a 10m topobathy surface.


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Fig 5 10m topobathy surface viewed in 3D.

The topobathy surface allows users to create seamless features across the coastal zone area

and perform analysis across the coastal zone with a common reference surface. To illustratethis point, contours were derived from the DEM across the entire coastal zone area.


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5m Interval contours and depth areas derived from the topobathy surface.

This case study illustrates how easy it can be with todays geospatial software to create atopobathy surface, but it also illustrates the challenges of applying a vertical datum shift and theerrors that can be introduced. While a topobathy surface was created, and seamless coastal

zone features derived from that surface, the accuracy and usefulness of the features was limitedby the resolution and accuracy of the input data and the accuracy of the vertical datum shift.

Ideally, bathymetry data would be combined with higher resolution elevation data (such asLIDAR) and a vertical datum model would be applied, taking into account various reference data(such as tidal gauge data). But in a practical sense, ideal data sources are not always available


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and compromises must be made. The key is to document the compromises and attempt to use

the best available data.

By fusing the best possible data from the land and sea in the coastal zone, stakeholders and

planners can make informed decisions and have the benefit of deriving new features that lieacross the combined surface. In the case study, this translated into a set of contours from sea

floor to mountain top. This topobathy surface could also be used to create inundation models,shoreline erosion analysis, or wetland habitat mapping.

Features and RepresentationsWith its production database technology, CARIS has adopted a data centric approach to

managing geospatial data and creating products such as maps and charts. By storing multiplerepresentations of the same database featurea technique that CARIS describes as one

feature one timevarious thematic datasets can co-exist without the need for storing duplicate

information.

This can be illustrated by the example of a lighthouse. For the user of a topographic map the

lighthouse is a landmark; for a mariner, it is a navigation aid; for the town planner it is a building;

for an aviator, it is a hazard; and for a military user, it could be considered a target. The CARISapproach to data management allows for the single lighthouse object to be stored in the sourcedatabase along with different representations so that the lighthouse can be properly portrayed

for the purpose of the end product. The images below show the differences betweenaeronautical, hydrographic and topographic products. All these products could be derived from

the same source.

In order to take advantage of the efficiencies that a data centric approach to data management

and product creation can provide, a flexible and comprehensive data model is required. The

data model facilitates the storage of all the geospatial features in the database and how featuresinteract. Data dictionaries describing features and their attributes need to be created; ideally

these data dictionaries should conform to international standards and can be a combination ofseveral thematic dictionaries to allow as many data types to reside together as possible. All the

features in the data dictionary will require symbols, line patterns or area fills associating withthem, depending on the geographic object type (point, line, area etc). The symbols used could

also change dependant on the type of product that this being produced by the system.

Using a data centric model allows source objects to exist with an endless variety of

representations, thus allowing the source data to be leveraged to create an endless variety of

data products. As more source data are incorporatedsuch as hydrographic, topographic,


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aeronautical, cadastral, environmental, or biological datamore better quality data products can

be produced.

A Data Centric Source Database can Facilitate Multiple Products

The merger of topographic and hydrographic data into a single database allows specializedproducts to be developed that contain a combination of relevant topographic and hydrographic

features (e.g. products for coastal management). This would be in addition to the standardability to create topographic map sheets or internationally recognized nautical charts from thesame production database.

Conclusions

The movement in the spatial community towards Spatial Database Infrastructures (SDI) haslargely been focused on the land data. This is changing partly because the management of thecoastal zone has become more urgent in the light of rising sea levels. Decision makers are

realizing that a clear picture of the coastal zone must include a combination of land and marinedatasets. Recent terms of reference from the International Hydrographic Office (IHO) Marine

Spatial Data Infrastructure Working Group (MSDIWG) states, There is a need to identify and

recommend solutions to technical issues related to interoperability between land and sea data,e.g. datum issues.7

The combination of existing map and chart data can provide a source of information for new

coastal zone products. When it comes to analysis and modeling, the creation of a seamlessDEM from mountain top to sea floor is seen as essential. If high-resolution height data, suchas LiDAR, can be merged with high-resolution bathymetry from sonar systems, and the zone

between the two is populated with data, then robust analysis is possible.


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The major challenge in combining land and sea data is the determination of a common vertical

datum reference, as land and sea datasets are traditionally referenced to different verticaldatums. Several organizations are attempting to meet this challenge by standardizing on local

or regional vertical datum models, but these models only cover small areas of the global coastal

zones. An alternative approach may be to store datasets in an ellipsoidal reference, and thenallow users to transform data to the vertical datum of their choice.

Data centric software solutions can provide a mechanism for storing land and sea features in a

single database, and can therefore facilitate the production of coastal zone maps thatincorporate the relevant topographic and hydrographic features.

The ability to create a topobathy DEM provides the ability to create continuous contours acrossthe coastal zone, contours that can reside in a data centric database. The seamless DEMs can

also serve as the basis for subsequent analysis relating to tsunami impact or coastal erosion.

The technological functions are available to merge land and sea datasets, but before doing soquestions surrounding coordinate reference systems, and in particular the vertical reference,

need to be addressed to ensure that the topobathy DEMs are created in a useful way.

1NOAA Vertical Datum Transformation, http://vdatum.noaa.gov2Adams, Ruth; Iliffe, Jonathan; Ziebart, Marek; Turner, Jim; Oliveira, Joao; Joining Up Land and Sea:

UKHO/UCL Vertical Offshore Reference Frame; Hydro International, 18/05/2009, http://www.hydro-

international.com/issues/articles/id696-Joining_Up_Land_and_Sea.html3Masry, Mark: Collins, Corey; Scaling Bathymetry: Data Handling for Large Volumes; Shallow Survey 2008

conference; http://www.shallowsurvey2008.org/images/stories/abstracts/28_masry_data_handling_for_large.pdf4Canter, Peter; Lalumiere, Louis; Hydrographic Surveying on the Ellipsoid with Inertially-Aided RTK; US Hydro

2009, http://www.thsoa.org/hy05/06_1.pdf

5UNB Ocean Mapping Group, http://www.omg.unb.ca/omg/6Service New Brunswick Geographic Data & Maps; http://www.snb.ca/gdam-igec/e/2900e_1.asp7IHO MSDIWG 2ndMeeting Terms of Reference, Sept. 10-11, 2008, Monaco, http://www.iho-

ohi.net/mtg_docs/com_wg/MSDIWG/MSDIWG2/MSDIWG2.htm

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GIS_Merging_Land_Sea_Elevation.pdf