Unit conversion (Maritime Charting)

Many nautical charts have elevation units in feet and depths in units other than meters, such as fathoms and fractions, fathoms and feet, and feet. When this occurs, use the units shown on the chart to collect your elevation and depth attributes. What you see on the chart is what you enter as your value when attributing a feature. This makes quality control more efficient and accurate.

Converting units

Once a chart is ready to be edgematched or converted to VPF, all the units should be converted to meters. This allows accurate edgematching.

Once you convert your units, you are not allowed to convert those features again. With the Units Conversion tool, you can convert based on your Dqyarea extent or a selected set of features.

Original unit values are moved to a field called orig_<item> in the feature table. You can label this field in ArcMap for visual quality control purposes. All the orig_<item> fields default to -99999. If you import your data with the VPF Importer, they default to 0. Once converted, the Units Conversion tool will not run on features with orig_<item> not equal to -99999 or 0.

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If your elevations are in feet, all features with field HDH are converted using Standard Conversion Table 4. All other elevation fields use Standard Conversion Table 1.

Feature class	Elevation fields	Depth fields
AqueductL	OHC
BridgeA	MVC,OHC,SHC
BridgeL	MVC,OHC,SHC
BuoybcnP	EOL
CanalA	WID	HDP
CanalL	WID
DangerA		HDP
DangerP		HDP
HazardA		HDP
HazardP	HDH	HDP
Hydarea		CVH,CVL
Hydline		CRV
IndustP	HGT,ZV2
IndustA	ZV2
LightsP	EOL
MaritimA	WID	HDP
MaritimL	WID	HDP
MisciwyA	WID
PipelinL	DEP,HSB,OHC
PowerL	OHC
ReefA	HDH	HDP
Relline	ZV2
Relpoint	ZV2
RouteA		HDP
RouteL		HDP
RuinsA	HDH, HGT	HDP
RuinsP	HDH	HDP
SnowiceP	HGT
SoundP	HDH	HDP
SepartnA	WID
SepartnL	WID
SweptA		HDP
SweptL		HDP
TunnelL		HDP

The following table describes the fields that are converted for each feature class listed.